Fistly make sure your front wheel is in sqaure, if it is then send it back and get a replacement rack if the replacement is also skew then it's most likely your drop out braise ons that are slightly bent. If you need to staighten it then I'd suggest seeing a frame builder who has the right tools for the job as you could been twisting and a bending a few time as you don't know which axis it's out in and there's a licklyhood you could snap it off. I've work in the cycle trade for a while and there is nothing like the right tools for the right job.