What I do is this. First, in Acrobat, zoom right in on the pdf to make sure the graphic in it is indeed originally a vector graphic. If it pixelates, it's probably a gif or other raster file in the original. If it is a vector, then I save the pdf. I then open up Illustrator and open the pdf. File>Open It comes up with a box "Acrobat pdf plug in". From there I pick the page I want to get the graphic from. That's about it really. Maybe you're missing some of the software. Follow those steps and once you open it it illustrator, it will work like any other AI file. If it is indeed vector.