First of all, some of you simply aren't helpful...just return it...indeed!
My son just got a phone (Pantech Matrix), and we were trying to transfer his playlist from Windows Media Player to the phone. Had to buy a SanDisk microSD card for the phone to hold the music (and also because nobody in town had a Pantech compatible USB cable to transfer the few songs he has). I bought a 4 gig SanDisk microSDHC card, and the store clerk told me the HC was just how they (SanDisk) classify the high capacity disks and nothing else was different. Well, folks, that is just plain WRONG. One other, critical, distinction is that if it says HC on it you must have a reader that can handle HC. My old (4 years?) SanDisk card reader (Imagemate 8 in 1) is not HC compatible, and so, even though made by the same manufacturer, it CANNOT read the card I bought. So, my two choices are: first, return the 4 gig SDHC microcard and get a smaller card that is not HC, which obviously limits the stuff he can store on his phone, or second, buy a new reader that can handle the newer HC disks. Some of you may have other problems (For instance, flipping the switch on the left side of the SanDisk Adapter, which somebody referenced and I tried, since nothing in the packaging mentioned that switch!) that aren't impacted by your reader, but check your device (camera, reader, etc.) and if it is over a few years old I wouldn't be surprised if it isn't modern enough to recognize an HC card (it may have HC stamped somewhere to indicate it is capable), and you have to decide if you want to dial backwards in terms of storage capacity or bite the bullet and upgrade your device to handle these advanced cards.