opened dwg file measurements not correct

I have opened a dwg file sent by a customer and I am attempting to add measurements to the drawing. (mac version of LibreCAD) I am a total newbie at anything CAD.

How do I make the measurement match on the drawing what the actual measurement is?

I used the horizontal tool, snap to entity and set the units to inches....but what shows is nowhere close to what measurements should be.

Hope this makes sense