Any know how to remove / add an Add In (.xla) using VBA or anything else

If you are trying to add or remove an addin from code then you need to use the Addin.Add method and then set the addins Installed property to true. This is all covered in the Excel VBA Help - if you just type addin in a VBE code module and press F1 with the word selected this will take you to the help topic.
To install or remove an addin directly from Excel use the Tools|Add-Ins... dialogue box available from the Excel Worksheet (not the VBE).
Hope this helps.