Normally in Ubuntu you can just start any app from the Unity dash, then when it appears in the Launcher you can right-click its icon there and set "Lock to Launcher".
But the process is a bit different for shell scripts, and pycharm runs from a script, pycharm.sh. You'll need a .desktop file. For a new installation you can create pycharm.desktop from scratch; or if you're upgrading from 2.x you probably already have a pycharm.desktop somewhere (in my case it was found as /usr/share/applications/jetbrains-pycharm.desktop).
Create or edit pycharm.desktop or jetbrains-pycharm.desktop to read:
Comment=Develop with pleasure!
[replace the paths shown above with the correct ones for your system]. Now having set this up, you can search for it with Unity Dash and drag the icon from there to the launcher, or just run it from the dash and then right-click it in the launcher and set "Lock to Launcher". All set!
If you're upgrading, your old jetbrains-pycharm.desktop will have the old version's paths in it and all you should need to do is update the paths.
Happy to hear improvements on this if anyone has them.