If you changed your computer and want “your PL/SQL developer login history and preferences” as well in your new computer. Copy the following folder to your new computer in the same path format!
C:\Users\[username]\AppData\Roaming\PLSQL Developer
If you need only the login history, open the following file:
C:\Users\[username]\AppData\Roaming\PLSQL Developer\Preferences\[username]\user.prefs
In the file, copy [LogonHistory] part to your new user.prefs file. It’s encrypted but it does not matter. PL/SQL Developer will dencrypt it 
I need this, because I deal with a lots of database with a lots of schema.. It is really hard to remember the passwords sometimes, so i let the PL/Developer to remember them (Preferences > Login History > Store with Password)
What if your old PC was Windows XP?
- Bob
search for user.prefs file in your application setting folders then apply the same trick
Found it in XP under C:\Program Files\PLSQL Developer\Preferences\