Example project showing usage of the NSPopover+Message category.

[NSPopover showRelativeToRect:[theInput frame]
                       ofView:[theInput superview]
                preferredEdge:NSMaxXEdge  // Show the popover on the right edge
                       string:@"Once upon a time, in a land far, far away, there was a little cake. It didn't last long."
                     maxWidth:250.0];

As described here: User Feedback & Simplifying the NSPopover.