Go to help, debug window, console:
Type: generate 1 and press enter.
You should mine one block! It might take some time if there's not many people on the network. Once a few people join, this will stop working and you'll to mine using a pool. There are no pools setup currently, but once we have some setup they will be listed here.
Disclaimer: I am not responsible for any damages that may occur from using this software. Use it at your own risk.