v0.3.2 will not be able to mine on the fullnet. You need v0.3.3.

Mining has been disabled from the UI, but can still be done using the "siac" command-line utility packaged with Sia: https://github.com/NebulousLabs/Sia/releases/v0.3.3
To CPU mine, first run "siad", then run "siac miner start [threads]" from a separate terminal.
Unfortunately siac will not display your CPU hash rate, but you can find it by viewing to localhost:9980/miner/status in your browser.