According to this link, see the photo in the link, the Dragonflies will work but you need the Camera Connector Kit.
http://www.wifihifi.ca/LatestNewsHeadlines/Hands-OnReview:AudioQuestDragonFlyBla ck1.5/Red.html
If a headphone and DAC combination have "made for iOS" or "made for iPad" then be assured that they will work as soon as you open them.
Any other item will need the Camera Connector Kit only if the DAC/amp draws 100mAmps or less. Otherwise it will need a powered USB hub which MAY add noise to the digital signal going to the iPhone or iPad. Even though it may need a specific version of OS/X or Windows it may work with the CCK from your iDevice.
Before you spend almost $200 or MUCH MORE for a DAC/amp like MicroStreamer, Audioengine, Dragonflies, Meridian, OPPO, SONY, Teac, ONKYO listen to low impedance, 32ohm, headphones first. Then listen to the DAC/amp on your home computer. Compare the sounds and then decide on how much to spend.
The Best Buy Guy needs more experience. The camera connector kit will connect USB keyboards directly and many other USB devices directly or with a powered hub. I have used my iPad to connect to my home stereo systems desk top DAC.
You have MANY more choices than I mentioned above. Read the reviews from reasonable blogs and publications like TheAbsoluteSound, Stereophile, and CNET.