iso's are imported as data. You can find them under documents.
To get them to appear under video you can select them all in MC and change their
'Media Type' tag to Video.
It may be a moot point (the OP may be gone), but that does not work for the OP, since he specifically stated that he has audio CDs as ISOs.
In fact, this is a long-term problem in applying these workarounds - ISO files are just images of disks, and therefore they can be either audio (CD or SACD), video (Blu-Ray, DVD and even VideoCD) or audio in video format (Blu-Ray Audio, DVD-Audio). Then there are also ISO files that are images of CD-ROMs and other data disks - which might contain Images.
All this fits in the category of "Things that most users don't usually use, but MC should support in the name of compatibility with all audio and video formats" and also the category of Somewhere On The Long-Term Project List.