So the issue really i sthat you cannot find/get Omnisphere to open in Mainstage?
You load Omni as you would any Instrument plugin, by click-holding on the I/O slot of an Instrument Channel Strip:
You'll get a list of 3rd party AU plugins at the bottom:
If it does not show up there, you may have some issues with your installation:
Does Omni work in both Logic and Live an the same Mac?
Go to Mainstage:Audio Unit Manager: does Omni show up in here at all, and as 'successfully validated'?
If NO: go to Library:Audio:Plugins:Components and check that Omni is actually installed
Then run Mainstage in 32bit mode to force the AU Manager to re-check all the plugins.
If that does not fix it, there will be some more work to do.