User form with formulas (excel 2007)

You don't need a Userform which requires VBA coding.

All you need to do is use Data Validation with the List Option and use a list that is based on a dynamic named range that encompasses the items in column-A on the "Data" sheet.

The Data Validation will provide a drop down to allow the user to select from the list of choices.

As for the "Enter B" cell: this is simply an input cell. But if you want to make sure that only numeric values are entered you could also use Data Validation in this cell using either the Whole Number or Decimal option and then specifying the min/max values.

Thank you very much