Where? In a query, report, form? You'd have to tell us what you are trying
to accomplish.
If you have two fields that you pull to a query and you want to add them
together, then you'd add a new column to your query and put something like
the following in it...
SomeNewName: [Field1] + [Field2]
For a report or a form, you'd create an unbound text field with something
like the following...
=[Field1] + [Field2]
You would not store a calculation in your table. That would be redundant
and would lead to problems if someone updated one of the fields, but forgot
to update the calculation.