Recently I have been working on a Sharepoint 2010 and I had created a Class Library project that will contain some classes and methods. One of the custom webparts I had created was calling a method from one of the class, but when I press F5 to deploy and test my web part I got an error and my assembly file was missing in Sharepoint. I realised that my Class Library assembly file was not deployed.
Below is a simple step by step instruction on how to include a assembly file in your Sharepoint package.
For example here I have a solution with a Sharepoint 2010 and a class library project. In the Sharepoint 2010 project there is a package folder containing a file called Package.package.