Head over towards the flower shop. There should be 2 guys standing there, and talk to them. One will tell you that his buddy went to get unifroms and hasn't returned yet. Go to the underground tunnel where the photobooth is, and a guy will ask if you're new and will give you one. The phiotobooth tunnel thingy is right to the left of the renaming tent.,
You first head down to the Underground. After, a Rocket Grunt spots you and assumes you're new. He flops on a black uniform and you head out to the Radio Tower. After, you show your nice uniform to the other Rocket Grunt which is blocking the staircase, he lets you in. Eventually, your rival spots you and rips off your uniform and shows your identity. But still... The Rocket Grunt lets you past.