I am not sure what help  you need.

In essence you need to:
Put the formula in the third cell  on your spreadsheet to add the first two
cells
Open the VB editor
Insert a userform
Place three text boxes on the form
Place a button  on the form
On the click action of the button
-set the value of the first cell to the value of the Textbox a
-Set the value of the second cell to the value of Textbox b
-set the value of Textbox c to the value of the third cell

Please come back to the list if you have any specific problems with this.
Tell us what you have done, show us your code, and tell us what the problem
is.