We are using an <iframe> on a page and when opening a dropdown menu, it blows away the iframe page. The iframe shows a "where to buy" map feature.
 
How do we change the vertical menu dropdown to one that spreads the items horizontal in one line?
 
Is there a tick option for this we have missed, or is the change not doable?

You asked this a while ago, so may have gotten it. make sure there is no link activated just upon clicking the dropdown.
Other than issues like that, if you can use a java menu, you could position it anywhere, in any orientation, just have to make sure it floats above the frame. You may just dump the frame, and use an image based javascript fly down, that activates on mouseover of the top image.