You will need the source code version of BE to get the file you are looking for, which should contain 3 main sub folders: BlogEngine.Net, BlogEngine.Modeling and BlogEngine.Core (or DotNetSlave.BusinessLogic in the most recent versions of BE).
Normally it's only the contents of the BlogEngine.Net folder that end up on the server, which explains why you would not be able to find the file.
Your best bet might be to go to the 'Source Code' section at the top of this forum and find the version that matches your version of BE and download it to a suitable location on your local machine.
Once downloaded and unzipped, look in either BlogEngine.Core\Web\Controls\ or DotNetSlave.BusinessLogic\Web\Controls\ for the required file.
Microsoft Visual Web Developer is free to download, you could use this to open the project file (of type Microsoft Visual Studio Solution) and play around with the code offline (all relevant code should be visible from the Solution Explorer in Visual Web
When you are happy with the changes and assuming the builds go well, then if memory serves me correctly you will need to swap the Bin folder on the server with the Bin folder from your offline BlogEngine.Net folder.
Hope this makes some sort of sense... don't forget to backup the current install.