I'm not sure what version of AutoCAD you're using, but I think the default setting for AutoCAD mechanical is to click and hold the middle mouse button (if your using a roller type mouse). You may have to mess around with the system mouse settings as well as the AutoCAD control settings.
M using AutoCAD 2008. The scroll wheel is not working for PAN.It is showing some commands options. I guess it must be some software settings...but couldnt find it. I tried using MBUTTONPAN command, it gives me PAN movement with scroll wheel but for only current drawing. On taking new dwg the PAN gets over. I wish to have permanent movement on my mouse click.
mbuttonpan. set to 1 for click-pan, 0 (zero) for popup osnap menu. I haven't used pan mode for years, once I learned I had osnaps at my fingertips for precision work. but each to his own. good luck !