[Rey_a]

No idea what a Boss looper RC-600 is ...

1) I have used a MIDI-cable with a USB-adapter for the tablet for exchanging MIDI between MobileSheets and a Yamaha keyboard. So I guess it should work with the looper as well.
2) There was no need to configure a MIDI channel on the tablet.
3) You need to find out which MIDI messages the looper requires to perform what you would like it to do.
And then enter them in MobileSheets
In 'settings' / 'midi settings' you can specify a midi channel to be used by MobileSheets.
In 'settings' / 'Midi connections', you can choose a MIDI library to be used by MobileSheets. I have always used the default, but some people needed to switch to an aternative library to get things working. Start with the default one, and choose an alternative when things don't work.

And finaly, you need to add the required midi action to the songs.
To to that, open the edit field for a song, choose the MIDI tab, choose the + sign to add the midi string, and verify that 'send on load song' is switched on. that will trigger Mobilesheets to send out the midi string you specified, whenever that song is loaded.

Good Luck!

Edit: oops seems we were posting at the same time.
Glad you got it working!