Hi, I have a problem!

I have created a normal excel-workbook containing some macros. In this book i put in two values in two different cells and then the answer is automatically calculated and displayed in a third cell.

So far everything is fine.

Now to the problem:
I would like to create an add-in based on the worksheet described above. I have been trying for a while but I don't know how to get the add-in function to use my workbook to do the calculations.

How can I make the add-in function to put data into and retrieve the calculated answer from the the workbook?

Thankful for your help!