And today's award for bad advice from a guy at a music store goes to....the guy above!
You don't want speaker cables--you want mic/line level balanced audio cable. The output of your mixer is
standard line level. The amplifiers are actually built into the speakers. Using unbalanced speaker cable would add (probably badly) to the noise.
What sort of noise is it? If it's just a general electronic hiss, I'd be experimenting with your gain staging. Depending where things are, try turning down the mixer a bit and turning up the speakers--or, if the speakers are all the way up, turn them down and add a bit more gain at the mixer.
However, if the noise involves some 60 cycle hum, try using an extension cord so you can run the speakers and mixer off the same socket--and do a search on this forum for "earth loop hum".
Anyhow, the one thing that will NOT fix this is changing to speaker cable.
Edited to Add: Your Mackie speakers actually have a facility to let you plug a dynamic mic straight into them. The gain setting (+36dB) is achieved when the gain control is turned fully clockwise. The maximum you should be using for the line out from your mixer is the 12 o'clock position which equates to +4dBu. This is a kinda silly way of doing things and could certainly result in you hearing noise if your speaker controls are turned up beyond half way.