First off, if you're importing MIDI files into MULAB Free you'll likely trigger the multi-channel per part threshold.
Second, MIDI files contain no sound - you need to supply the sounds some how. MULAB comes with a vast array of built in sound generators you can choose from and there's an even wider array of free VST plugins available (see the KvRAudio plugin database). So that part you'll need to decide on.
Once you have that set up, it's "Session", "Mixdown Audio" (which I guess you tried already).