To link a project to a resource pool, you must have both files open at once. Then, in the main project, click the Tools menu, choose Resource Sharing, and click Share Resources.

Then you will see this dialog.

This lets you choose what resources to use, and what to do when a conflict occurs (to assign the resource to the sharer or to assign it to the pool).

Once you click OK, the sharing will begin. Note that sharing goes both ways: resources in the main project file are added to the pool, and resources in the pool are added to the main file.