First, render your beats to an audio format 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 File menu and select Add Sounds. Browse to the location of your saved Beatcraft audio file and select it.
That's all there is to it!