See Hardty ?
By the number of messages asking what you want to do exactly, it sounds like YOU did not manage to let us know what exactly you wanted to do, and that's why you needed to do more analyse and be more precise about your intentions.
If you mean a popup like a simple message dialog, there is the Browser.Alert action.
If you mean a popup within your game, as xanxion said, add a layer and make it visible/invisible.
If you mean a popup like a new browser window that pops on top of your current game, it's not really doable or something you'll want to do.
As mentioned in my previous message, just see what are the tools at your disposal.