MS Excel Add-In

Can someone supply an example of an MS Excel Add-In module? I would like to write one to automate some financial operations such as weighted market average. Naturally the add-ins that are written by MS are password protected, and I cannot take an example from them. I would like it to behave like the VLOOKUP function when one types that into the formula bar (eg. implement help for the drag & select boxes that pop up). Please help me or point me to a webpage that allows me to view an example of such code. Thank you....

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