I'd like to add some reverb to the singer's mix in the cans for the vibe as he sings to instrument tracks, but not record the effect. My interface is a Focusrite 2i4 and I'm running Sonar Producer. I'm new at this so thanks for any help!

I create a bus called reverb and put reverb in the FX bin.  Then I send the vocals to it (add a SEND to the vocal track).  It doesn't record, but you and the singer can hear it.  That is also how I add reverb at mix time.
 
1-Right click on any bus and select Add Stereo bus.
2-Right click in the FX bin of the new bus and insert whichever reverb you like.
3-Select the vocal track and click the + but Sends.
4-Select the us with the reverb as the place you send the vocal track.
 
I recommend you set the reverb in the bus to 100% wet and 0% dry.  Then in the vocal send, you will probably want to turn the send amount quite a bit.

Thanks! Where does the bus 'out' get routed to direct that wet signal?

Billy86
Thanks! Where does the bus 'out' get routed to direct that wet signal?

Send it to "Master" (that's the default setting, so you shouldn't have to do anything).

Billy86
Thanks! Where does the bus 'out' get routed to direct that wet signal?

Depends on how you've got your monitoring & headphones set up.
 
My phones are fed from outputs 3/4 on my interface, so I create a headphones bus in Sonar, output routed to H/W outs 3/4.
 
Then, everything I want the singer to hear is fed with a send from the relevant tracks/busses (nearly always busses)

Here's a previous thread.  I use the method described by Sacalait.
 
http://forum.cakewalk.com...h-reverb-m3090925.aspx

Because the 2i4 does not come with the GUI mixer, you will have to activate the new vocal tracks input echo. This will send the signal to the master output to mix with your playback while recording. But the downside is now you will hear the latency of your system, which varies depending on buffer settings etc. 
This will result in a varying degree of slap back echo, which to some is OK if you mix it in properly. 
Buy turning the tracks output level down you can control the amount in the playback mix. 
Crank up the send to the reverb and so on. 
A tracks level controls have no bearing on the recorded level. The audio interface sets that. 
I use a small mixer with built in reverb to solve the problem.  For $100 they are very handy little things. 

Hmmm. Not having any luck. I'm sending from the Vox track to a Reverb stereo Bus (post fader button lit, and have a healthy send level), and I have a reverb plugin in the effects bin (using a  preset Small Hall) of the the reverb bus. Bus is going out to the master. The reverb is working on the recorded vocal (which I don't want). But I can't hear it on the vocal in the singer's headphone mix??? Trying to get a wet signal in the headphones, and lay down a dry signal.
 
I have an external mixer with some effects built in. A Yamaha MG82cx, but I don't know how to get it talking back and forth with my Focusrite 2i4.

Amazing. That's the same mixer I have. 
Simple; 
My 6i6 and your 2i4 has 3/4 out so I use that to the mixer. 
I have a Joe Meek pre amp which has 2 outputs so one goes to the mixer and one to the interface. 
You could do the same with anything that can split an XLR cable. 
But what you can do is run the headphone output of the 2i4 to a second channel of the mixer. 
So in other words 
2i4  1/2 out goes to studio monitors. 
2i4  3/4 out goes to a stereo input of Mixer- 3/4 or 5/6 using standard patch cables
2i4 Headphone out goes to 7/8 stereo channel of mixer via 1/4TRS Y cable to 2x 1/4 or RCA
This is a standard cable available even in dollar stores. They come with 1/8" TRS but the 1/8" to 1/4" TRS adaptors are also common. 
You use the mixers headphone jack,, this is bonus as it's much louder too. 
Now turn the direct monitor control to "input" 
You will now hear just the vocal input in channel 7/8 you can add the mixers reverb. 
The playback level is now controlled by the other channel 3/ or 5/6
 
I'm not sure with yours what happens when you turn the big knob down. on mine the monitors are off but out 3/4 is still hot. 
 
 

Billy86
Hmmm. Not having any luck. I'm sending from the Vox track to a Reverb stereo Bus (post fader button lit,

I believe you need to set the reverb send using pre fader, not post. You would need to switch to post when you want to listen back. At least that's what works for me.