Loading A Project
Mixcraft can load Mixcraft project files (MX4, MX3) by clicking the File menu and choosing Open Project... (Ctrl+O).  Navigate to your project file, select it and click Open. 

Alternatively, you can click the Open Project button on the toolbar or click Browse... on the New Project Dialog

The software can also load Mixcraft 2 project files (MXC) and Mixcraft Templates (MX4Template files).

Loading A Standard MIDI File
Standard MIDI files contain MIDI data on multiple tracks and Mixcraft will import them.  When it creates a new track, it will use the default General MIDI synthesizer as set in the MIDI preferences.