External Effects Sends - Focusrite Scarlett

the 2i2 will not support external hardware effects sends, in fact, even the Saffire PRO doesn't do external sends.
 
what exactly are you wanting to use external sends for?  there's probably an easier and cheaper way to do it.

you can do that without a send/return the send/return would be a quicker way to do what you want, but there's a cheaper way!
 
record the guitar dry
set up cables from the output of your soundcard to the input of the guitar FX unit
connect cables from the output of the FX unit to the INPUT of the soundcard
set up a new audio track in MC with the inputs set to the same inputs as your cables are sent to
solo the guitar track if you have anything else in the project
arm the track and hit record.
 
the guitar track will go out to the soundcard, into the FX unit, out of the FX unit, into the soundcard, then onto the new track.
now you'll want to mute the dry guitar track so that you have only the "wet" one in your project.

Any way I can silence the dry recording playback so as not to create an echo loop? 
Thanks

Not tried the method yet - I was wondering if a loop would exist given that it is playing and recording the same sound simultaneously but i guess not.