Parallels tools install-can't click ok

Upgrading on Snow Leopard to latest version of Parallels 4 from previous P.4 build. When installing Parallels tools, I get a box saying digital signature not found...with a button to click to proceed anyway, etc. But when I try to click in the "Yes" box, nothing happens, I lose the mouse, have to press contol-option to regain the mouse but nothing has happened. Have tried various keyboard alternatives to mouse click. Nothing gets me past this dialog box. Very frustrating...