Yes OK its a gantry router so it is the tool that moves and not the table, so the X is actually the wrong way. What you need to do is find the setting for reversing the motor. As I said its been a while since I had Mach2 Installed so I cant remember where it is or actually what it is. I am not sure if there is a specific setting for reversing or whether its just the low active you need to check or uncheck, you will just have to look through the settings from the config menu.
Also you wont find the set steps button i mentioned in my previous post, sorry, I forgot it was Mach2 you were using, its only in Mach3 I think. You will have to work out the steps per unit yourself by TPI of ballscrew, steps of motor, drive stepping and motor steps per rev.
Incidentaly why are you using Mach2? might be the time to install Mach3 and try it out, you can have Mach2 and 3 on the same computer so if you find Mach2 is better you can keep it, but I think once you have tried Mach3 you will stick with it

Hood