Bill

Not sure what you've been doing but if you are trying to move one vert to the position of another ---- activate the move tool then hold down the 'v' key --- hold down the left mouse button and drag the vert towards the other ------ the "moving" vert will snap to the other. Then, deselect the moved vert ---- then select both verts and merge verts.

If you want to merge verts that have some distence between them you simply need to change the tool settings. The middle of the three buttons on the extreme upper right of the screen (on the "status line") will access these settings. Just remember to reset to default if you use a large setting - you might start merging hidden vert, on the other side of your model by accident.

Hopefully this might help you.

good luck

Keith