Um. I found it pretty easily: http://msdn.microsoft.com/en-us/library/system.collections.collectionbase.aspx
And it lists the Add method under "Explicit Interface Implementations"
How did I find it?
I Googled for CollectionBase class
] by highlighting it in your question and right clicking. Select "Search Google for..." and it was the top hit.
Perhaps your Google-fu needs a little improvement?