Use the little "link" icon (looks like two links of a chain) while the button is highlighted. You are prompted for the page to link.

In case you're new to this ... David's right, but here's a bit longer explanation.
If you left-click on your button (or item), that highlights it. Somewhere on your page is
a black window that has the "properties" for your button. On the upper right of that window
is an icon that looks a bit like a chain link. Left-click on it. You'll get a new window opened to
let you specify where the link goes to.

There are two basic kinds of links you can setup: Web URL, or wireframe page. The former
lets you enter arbitrary URLs. The latter lets you specify one of your iPlots project's pages.
Once entered/chosen, click on "Save".