The FlashForge 5M Pro has automatic calibration. Follow these steps to calibrate:
1. Select the "Settings Cog":
2. Select the 4th tab along and check the "PID calibrate" checkbox
3. Hit the "Start" button:
If prints are still not sticking to the bed, try using the included gluestick on the build plate or see this article for other reasons why your print might not be sticking.