That's a weird alert. That alert is not put up by FinderPop and looks like it comes from the system, but it says "the FinderPop preferences pane" instead of the official title "preference pane" like the dialog below it (note the extra 's'.)
Does it matter whether you choose "Install for all users" or "install for just you"?
Sometimes error information will appear in Console.app - it might be worth opening /Applications/Utilities/Console.app and seeing what appears when you try to install FinderPop...
The best version of FP to run on 10.4 and 10.5 is 2.2.1 beta1 -- see viewtopic.php?f=3&t=30
(It has a couple of thousand users and no-one's complained much so far... plus it's more compatible with the 10.5.8 Finder than the official 2.2 release.)