My bad! Turns out I was using a third-party USB cable that works with previous versions of Suunto Dive Manager but not with the latest version. I bought a new USB cable compatible with DM 5.0 and that solved the problem. Can't really blame Suunto.
I've used a Suunto Cobra2 for about 150 dives. Not a bad computer, but Suunto's support is absolutely awful. I recently moved from a Windows XP to a Windows 7 PC, and my USB upload cable stopped working with Suunto's Dive Manager software (latest version). It still works with DiveLog 5.0 (a third-party program that's pretty good but does not capture the RGBM and alarm data from the Cobra2).
I've been all over the Suunto website several times, sent them questions, and done extensive Web searches. It appears that Windows software for the USB cable is no longer available (although Suunto never gives a straight answer).
I'll keep using the Cobra2 as long as it lasts, but I'll never, never, never buy another Suunto product because of their worse than useless support.