excel training course london - Merge data from two sheets using dsum function?

I wish to add numbers from two sheets with same numeric referencing. I think I can use dsum function but am unsure. Perhaps you can point me to correct function and formula. Sheet 1) I have three columns, column 1 is static numeric reference, column 2 is name to column 1, column 3 is data which I wish to capture. Sheet 2) is sames format as 1). Many thanks in advance. Maria