If you click the "Send It Back" tab in RX5, it should take you back to SONAR where you see a dialog box saying "Select Process to Commit Changes" (or something like that, I'm away from my DAW). That will do it.
 
Sometimes the dialog doesn't appear and you have to repeat everything. Sometimes the modified clip will start playing on its own, which appears to be due to thru RX Connect plugin in the Effects Bin. I don't know why this happens.
 
The RX Connect route is a bit clumsy and not perfectly reliable in my experience. On the other hand, RX5 is fantastic.

Yesterday, I gave it a try and it worked for me.  I inserted RX connect in the fx bin, selected the audio clip, opened RX connect (it starts to retrieve the audio clip), clicked in "Repair" then RX opens the clip in its pool, made some tweaks and clicked in the Sonar tab to send it back.  Although I selected only the clip, RX brought the entire track to the pool but I don't think it's a problem at all. Well, I'm gonna do more tests but, at least, I think it works ok.

I am able to use RX5 using the RX Monitor method. I plan to try using RX Connect also and then select the method which I find easiest to use. However, under any circumstance, when I open RX5 within Sonar, an error window appears that I have to click on several times before it will disappear permanently. In a previous post above, msorrels indicated that this could be solved by setting the regionflag to off. In my registry, the regionflag is a DWORD set to 0x00000000. Is this off?

RegionFlag set to 0 is off.
 
You can read more about this in this thread:
http://forum.cakewalk.com/FindPost/3221385
 
If you are getting an error from RX with a caption of "Can't open use session data folder" and text like "The currently selected session data folder cannot be used.  Open preferences, and choose a new session data folder" that is the region flag problem. 
 
But if you error messages have anything to do with sound drivers or ASIO, that error is your audio card doesn't like sharing its drivers with two applications.  The only way to fix that (if it can be fixed at all) is very specific to the drivers/sound card you are using.  There is an option in the Sonar preferences about sharing that might help (though it works so rarely I'd not get my hopes up).  When that doesn't work you can sometimes set the external application (in this case RX) to use the WDM/MMO/Windows native drivers instead of ASIO.  So let Sonar have the ASIO drivers and the external app use the native windows driver.  That kind of setup is a very fragile and unlikely to be reliable but I have seen it work.  Those are the only two generic work arounds I know of.  Everything else is driver/audio card specific settings and other configuration magic.

Is this only an issue with the newer RX versions? Because that would be a good reason NOT to update, for me. I'm on RX3. Whenever I close RX3 after editing a file Sonar sent, Sonar says "this file has changed, would you like to update the waveform preview" or something like that. It always goes automatically.

I believe that is the case. Sharke just asked similar with using Adobe Audition, and the above is also what I observe when making destructive changes. I "believe" the above RegionFlag reference is for programs which can see a "clip" discretely (i.e., "Region FX") inside the underlying wav file, and that both older versions of RX and Audition do not have this capability (so get the entire wav file passed to them instead).

Right, true, RX3 always gets the whole wave. I don't see this as a problem although it would be "neater" if it didn't. Still, bounce to clip is a perfectly valid workaround if you want to leave the larger wave untouched. 
 
My main point was that when the file comes back, Sonar does automatically register it has changed and updates accordingly. 

I got RX Connect to work with RX5
 
http://forum.cakewalk.com/Izotope-RX-5-and-Sonar-m3505341.aspx
 
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