OpenTK does not come with any installer or setup. Instead, you download the OpenTK binaries and add a reference to "OpenTK.dll" in your Visual Studio/SharpDevelop/MonoDevelop project. (Unzip the binaries first!)
It is a good idea to add "OpenTK.dll.config" to your project, and make sure the "Copy To Output Folder" (not "compile"!) is set to "Copy Always". The application will run without this on Windows, but not on Linux or Mac OS X, so please don't forget that!
Last, but not least, make sure the "Copy Local" property is set to true for the OpenTK reference, to simplify the distribution of your application.
It's as simple as that!