Before you go messing with barrel adjusters, you have to know WHICH WAY they need adjusted, in or out; the best way to find this out is to shift the right-hand shifter to "7" (high gear, smallest-toothed gear on the rear). Check the cable; if it's loose, you need to turn them OUT; if it's REAL tight, or won't even GO into 7th gear, you need to turn them IN. (in this case, "righty-LOOSEY, lefty-TIGHTY") Also, if there's a LOT of slack, barrel adjusters won't be enough to fix it.

The sad part is, most big-box stores don't have someone who knows what they're doing, and a lot of bike shops won't touch big-box bikes AT ALL; even if they do, it'll COST.

I could fix it in five minutes, but my Star Trek transporter's broken, sorry.