I'm new to Mach 3 and I'm having trouble figuring out how to do something I think is very simple..

I want to simply move along the X axis an exact amount of distance, for example, 5 inches.
For various reasons I often find myself needing to reference the center of something, or the top of a previous cut that I know where it is, in reference to my current location... So how can I put my router somewhere, and tell the machine to go 5" in the X direction without using the arrow keys and jogging it, getting it close, then slowing it down and bumping it into place?

I hope this is the silliest question anyone has ever seen and they can tell me how to do this simple task and I'm just overlooking an elephant in the room!

Frequently I will just enter the distance I want to move in the axis DRO as a negative (if moving positive), and then send the axis to zero.
John Champlain

You could MDI in incremental units, ie
G0 G91 X5 will move the X 5 units positive from its present position
G0 G91 X-5 will move it 5 units in the negative direction.

Always a good idea to follow up a G91 with a G90 to put it back into absolute mode  unless you always wish to be in Incremental mode.
Hood

Also another thing you can do is put it in step jog mode and enter 5 for the step value and when you press the Jog key it will move 5 units.
Hood