Your reply indicates that you have installed the Palm Desktop into a user account, not into the base account with administrator privileges. I don't recommend doing that, but you will find the equivalent folders in your configuration here:
Macintosh HD:Users:<username>:Library:Application Support:Palm HotSync
In answer to your other question, you cannot. If you seriously want to synchronize a Palm device under any version of Mac OS X 10.4 or later—including Mac OS X 10.5.x—you should instead install the
Missing Sync for Palm OS. While it is not yet certified for use with Mac OS X 10.5, it soon will be, and you can download a copy of the nearly ready beta release
here.