For MC to automatically detect that an album is a (Multiple Artists) album, all the tracks need to be in the same folder, the album tags must be identical for all tracks and at least one artist must differ from the others.
Assuming you have not moved the artist into the title...
Select all files in the album.
In the action window (lower left of the application) choose "Tag"
Look for the Album Artist tag... if it is not there, then right click anywhere inside the tag window, locate 'Album Artist' in the list under "Also Show" and click on it.
Now enter (Multiple Artists) into the Album Artist field and press enter.
The steps above should be enough to bring all the tracks under a single thumbnail in your list. If you'd like to get them all filed away into a single folder, right click on the selected files and use the "Library tools > Rename, Move & Copy files" tool to do that.
-marko