It's not essential to lock your picks - any unlocked picks will be automatically locked at kick off. Locking your picks is only required if you'd like to see what your fellow pool mates have picked before the start of a match.
If you'd like to view other people's picks, try opening a pool, click on the Matches button within the pool (this is where you go to view what other pool members have picked) and select an upcoming fixture. Below the fixture and your pick you'll see an option to lock your pick. Click on that to lock your pick for the relevant match, however, please remember that locking your pick is confirmation that your pick is correct and final, so you can't make any further changes to your picks once you have locked them.