There are three parts to a query formula. The field name, colon, and the formula. From the query design view, click in the next available blank field. Then type what you would like to name the field. Follow with a colon, then the formula. User defined field names have square brackets around them.
A view from the Query design will look like the following screenshot below.
Another option is to right click the formula field in the query design view then click build. This option is nice because you can double click on the field to insert it in the formula. It also helps avoid typo mistakes.