I'm tired of having to look up the query syntax every time I need to do it, so blogging it:

1. Start ldp.exe
2. Connection | Connect and choose your DC
3. Connect | Bind and authenticate
4. View | Tree and browse to the top-level OU from which you want to search
5. Connect | New to clear the right pane
6. Right click on that OU and choose Search
7. To search on the alias, use: “(&(objectclass=*)(mailnickname=aliasnamehere))“

More information: http://support.microsoft.com/?kbid=224543