I've had no trouble importing ebooks in a wide variety of formats including mobi. MC treats them as data (document) files. Make sure you have Data Support enabled in Options\General\Features.
If you have mobi associated with an external app then double clicking it will open it in that app.
MC cannot display ebooks, nor can it read or write ebook metadata (author, title, cover art, etc.). This is a shame since one of MC's key differentiating strengths is its ability to manage all media types. ebooks are becoming an important media type and they are hard to manage without using the metadata.
Until recently I managed all of my 10,000+ books with MC but recently threw in the towel and started using Calibre for non-pdf ebooks. I'd switch back if MC had even rudimentary support for ebooks since it is a huge advantage having all media in one library.