First, if the report is based on a query, you will have to make sure the field is included in the query results. Go to the query and add the field if it isn't all ready there.
Then, put the report in design mode, "View-Design View"
then click on the "Field List" icon on the tool bar. It looks like a little sheet of paper with lines on it. You can hover your mouse pointer over each icon to see the names.
The field list opens with the name of the source (query or table) at the top, and a scroll list of available fields. Scroll down to the field you want to insert. Click on it and drag it to the area you want it to be. You may have to move some other fields around to make room. Hope this helps!