In order to use an encrypted message base (that's the maion difference between home and pro) you need to create a backup, uninstall TB, install it again and while running it for the first time you select the checkbox that says that asks for an encrypted message base.
After that you restore your backup.
Changing the master password includes the same unistalling and installing, so you'd better be sure what paasword you want to use.

In case you're running TB from a non-admin account (as you should use every Windows program) the uninstall doesn't delete TB's registry settings. (No faiult of TB, that's the way Windows works.)
In that case you don't need to uninstall TB, you merely delete its registry settings.
Close TB
Start Regedit
Delete the registry tree starting with: HKEY_CURRENT_USER\Software\RIT\The Bat!
Close Regedit
Start TB
Now TB will come with a window that looks like this picture
My picture is in Dutch as that's my system language, but I've marked the box that you need to check.
Next it'll ask for the path to your mail directory, don't point it to the same path as your unencrypted message base, because that'll cause problems. (Unless you've moved/deleted the unencrypted message base)
Now restore your backup.