Under Mac OS X 10.3.9 and earlier with iSync 1.5 or earlier, you do not add your Palm device using the Add Device… option of the iSync Devices menu. Instead, you download and install the iSync Palm Conduit 1.2 from the Apple Downloads area, then launch the HotSync Manager [or use the HotSync menu of the Palm Desktop] to configure your conduit.
The iSync Palm Conduit appears in the Finder simply as Apple, and in the HotSync Manager as iSync Conduit. Double-clicking on it in the list will open a dialog box in which you choose the option to Enable synching for this Palm device. When you next launch iSync, your device will appear in the main window, where you can press the Tungsten icon to drop down a configuration pane to set the parameters of your synchronization.
Mac OS X 10.4 and later and iSync 2 and later follow a different procedure altogether, but the still do not select the Add Device… option in iSync. Instead, they select Enable Palm OS Syncing… from the same menu, and follow the instructions which appear. Part of that process is the installation and configuration of an updated iSync Palm Conduit for iSync 2 and later. It is not the same conduit that you need to download and install in your configuration.