In most cases an imported part will have an origin somewhere is space but almost certainly not in the correct spot. These steps should help you to move your part origin to the desired position.
Create a point where you want your part origin. The easiest way to accomplish this from a solid model is to extract geometry from the edges on the model. If you imported a geometry model the point may already exist.
Click on Modify-->Move Part Origin
Hold down the alt & shift keys and click on the point where you want your origin. ( Your cursor will become a square box. This is normal.) The X, Y and Z values in the origin dialog should change. If not, make sure the Origin dialog is the active window before performing the alt-shift click.
When the values in the Origin dialog are entered click the Do It button