link the iphoto library on my desktop to the dock iphoto icon

What is your iPhoto version? It will depend on the version of iPhoto that created the library and the version you have installed.

If the iPhoto library on your desktop has been created by iPhoto '11 and the Dock icon is linked to iPhoto '11, then simply double clicking the iPhoto library should launch iPhoto on it and open it. And the next time iPhoto will use this library automatically.

if the library has been created by an older version of iPhoto, hold down the alt/options key while you double click the iPhoto icon in the Dock. This will bring up the library chooser. Select your library on the desktop,

If the library has been created by iPhoto 7.1.2 or earlier, you need to download and run the iPhoto Library Upgrader to open the library in iPhoto '11.

See this link: iPhoto '11: About the Library Upgrader