You need a PDF writer installed on your machine. PDF995 is a free one you can get from the internet. Once installed you don't "convert" but "plot to" a PDF. When you get ready to plot you can use the plotter PDF995 and when it plots it will make a PDF.
I am running Carlson Survey 2008 on AutoCad LDD 2007. One of my plotter options is "DWG to PDF". It works great. Type "plot" and look under the available plotter list.