Are you using Mixcraft 7 build 235? If I select "Mix to Each Track To A Separate File," then click Edit Details, the separate files option stays ticked, and it does mix down to separate files. I wonder if you're doing something differently from me, because I can't get it to happen in Mixcraft 6, either.
Make sure you don't have a track soloed!
Here's what I'm doing:
1. Select "Mix Down" from the File menu.
2. Select MP3.
3. Put a checkmark next to "Mix to Each Track To A Separate File."
4. Click Edit Details and change some settings.
5. Name the track and Mix down.
6. I get separate tracks (I have 3 or 4 tracks in my project), and none of them are soloed.
Build 236 will add more time to the trial period (due out tomorrow).