I
believe there are different mini usb cables that will deliver POWER OR DATA OR BOTH.. You may have a Data only cord. I saw on Amazon and mini USB power cord for under $3.00 delivered. I know Walmart also sells
Power mini usb cords.
From what you say, you have a data cord only. On mine, when its plugged into the bike, on the second screen, the GPS shows a "lightning bolt" indicating its CHARGING. If you don't have that, again its likely, its a data cord only (for updates, etc)..
Amazon power cords:
http://www.amazon.com/Power-Supply-Cable-Charger-Garmin/dp/B00513ZHC2/ref=sr_1_1?ie=UTF8&qid=1339844381&sr=8-1&keywords=mini+usb+power+cord+garmin"]Amazon.com: Power Supply

Mini USB - Cable Charger Cord Lead Wire for Garmin GPS (5 Feet Black) - 200, 205, 250, 255, 260, 270, 300, 310, 350, 360, 370, 500, 510, 550, 600, 610, 650, 660, 670, 680, 710, 750, 760, 770, 780, 850, 855, 860, 880, 1200, 1250, 1300, 1350, 5000, 1260T, 1350T, 1370T, 1390T, 1490T, 200W, 205W, 215T, 250W, 255W, 260W, 265T, 265W, 265WT, 275T, 285W, 285WT, 465T, 755T, 765T, 775T, 785T, 885T GPS Cable: Camera & Photo
This link is a Garmin DATA cord only, NO POWER- its in the spec's:
http://www.amazon.com/Garmin-010-10723-01-USB-Cable/dp/B000BM8RFM/ref=sr_1_4?ie=UTF8&qid=1339844465&sr=8-4&keywords=mini+usb+power+cord+garmin"]Amazon.com: Garmin USB Cable: GPS & Navigation
The Powerlet cord I used:
The 015 cord is what I had Powerlet
modify for me (it came with the rt angle cord, the 12 volt to 5 volts adapter, NO COIL, I ahd them put heavier wire where I would be working, (the stock wire is too thin) and at the 12 volt end, I had them leave it bare ( I put on my own fuse conector and tapped into the ft running light):
Mini USB Short Powerlet Cable - Powerlet Products
It was a bit cheaper(about $35.00) as it didn't thave the fuse nor the 12 volt plug (you can see my pic's how I hooked up the 12 volt end)