This situation has been brought up many times before.
The simplest solution, although this will only give you the default settings, is to copy the aliases.ini file from the Defaults directory off of your installation directory to your installation directory.
You can determine your installation directory by typing
Code:
//echo -a $mircdir

After you do that, you may need to:
1) open your Remote Editor (Alt + R)
2) click the Aliases tab
3) click View - Nicklist
4) select File - Load
5) select the Aliases.ini file in the installation directory.
6) close the Remote Editor
7) restart mIRC