You can't directly load the Beatcraft project files into Mixcraft, but you can save the project as an audio file and load it into Mixcraft.
In Beatcraft, click on the File menu, select Render To... and then choose one of the audio formats (MP3, WAV, WMA or OGG).
Remember where you saved the file so you can locate it when you load it into Mixcraft.
In Mixcraft, click on the Sound menu and select Add Sounds.
Browse to the location of your saved Beatcraft audio file and select it for loading.
That's all there is to it!