When you submit a URL to BrowserLab, we post it to our browser servers which navigate to the page in the actual browser and take a snapshot of the page. Those snapshots are returned to the local client application and is what you view on your desktop. So the interactivity you have with the page being viewed in the BrowserLab UI is very limited. Really, the only interaction we allow is via links on the page. You should be able to roll over those in BrowserLab, and see a tooltip that details the link URL and should tell you how to follow the link from the UI (I believe it's Cmd-click on Mac and Ctrl-click on Win, but check the tooltip for sure). If you're not seeing that level of interactivity, there's a bug and we'd like to get more details about what you're doing and the URL you're testing.
Otherwise, for more interactivity, I'd recommend you use our BrowserLab for Firebug add-on. While the focus of this blog post ( http://blogs.adobe.com/browserlab/2011/08/22/testing-private-content-with-browserlab/ ) is about testing private content, it should have enough details to get you started with installing the correct pieces and with using them to interact and test your page in BrowserLab. Let me know if you have any further questions.
Hope this helps,