Numbers

Without a screenshot just guessing what you are trying to do, but you can do things like this:

Just select the columns (I selected one, held down the command key and selected the other) and drag the SUM token at the bottom of the Numbers window into a cell where you want the sum of the separate columns.

You can also select cells at the bottom of the columns and insert the SUM formula via the menu:

There are other ways too. If you are new to Numbers and you haven't already done so, be sure to go to Help > Numbers Help and have a look.

SG

Adding to what SG already posted.

you can use the function sum() which takes any number or arguments.

e.g.

sum(A1, Q2) will add the two cells A2 and Q2

sum(A, Q) will add ALL the cells in column A and Q

sum(B1:B10) will add all the cells between cells B1 and B10

sum(B1:B10, C3:Z3) will add all the cells between cells B1 and B10 and the cells C3 and Z3

sum(B1:B10, C3:Z3, R5) will add all the cells between cells B1 and B10 and the cells C3 and Z3 and cell R5