Adding external HD

I am pretty new to Mac and FCPX. I have been editing for a couple of weeks now and like the software. I am having an issue though and I cannot get past it. For some reason I cannot get FCPX to recognize my external HD. I took the whole lot to the Apple store yesterday and they helped me reformat my drive to the right thing. I think that worked. It still does not come up when I open FCPX. When I import it does not give me the option to select it. This feels like a very simple setting. Any ideas out there? Thanks.