I could be mistaken but I believe there were some files missing in the ST6 SDK distribution. I vaguely recall someone else noticing it previously. The samples provided there were copied from a release from https://solidedgesamples.codeplex.com that was available at the time. I would imagine there are newer releases since but most certainly if you download the latest release from CodePlex, it should work as expected.
In another thread, we discussed the open source stuff getting moved around. The source code for the samples has been moved to https://github.com/SolidEdgeCommunity/Samples. There you will find the latest code available. I am working on a CodePlex release this week, complete with documentation and videos. If you want to get a jump on the direction you're going to want to take (if you're doing addin development), you can download the latest source at https://github.com/SolidEdgeCommunity/Samples/archive/master.zip. Once unzipped, open ~\Samples-master\src\cs\AddIns\AddInDemo.sln. *Note the VB version has not been updated yet. You'll want to read the Readme.docx as well. I've put a ton of work into making addin development easy, especially compared to the current CodePlex release. The framework is now in place and I'm working on wrapping up everything else to go along with it.
If you don't want to mess with any of this until it's documented better, I'd understand. If you're like me though, you'll want to look at it.