This can be pretty tough. I can't tell from your post how much you know about Access, so we'll start with the simple answer, and just ask if you need more info.
Crosstab queries create their field names on the fly. I would recommend that you not base your report directly on the crosstab, but build a second query on top of the crosstab. Then you'll see how Access is naming those month fields. At that point, the fields in the report footer should just be set using the Sum() expression.
Start by playing with a query based on the crosstab, and perhaps you'll see what I mean. If not, could you post back with the name of an example field from that query, as well as names for all related controls and their control sources from the report?
Crosstab queries create their field names on the fly. I would recommend that you not base your report directly on the crosstab, but build a second query on top of the crosstab. Then you'll see how Access is naming those month fields. At that point, the fields in the report footer should just be set using the Sum() expression.
Start by playing with a query based on the crosstab, and perhaps you'll see what I mean. If not, could you post back with the name of an example field from that query, as well as names for all related controls and their control sources from the report?