Mark1982, you need to understand the two ways to SUM a field across a domain.
In the first case, you write an SQL AGGREGATE query where in the aggregate, you have a function like SUM, COUNT, MIN, MAX, AVG, and a few others. The SQL statement will include a SELECT clause that uses the aggregating keyword on a given field. There are restrictions on what other fields may be present because of the nature of aggregation. You will also have a FROM clause and possibly a WHERE clause.
In the second case, you call a DOMAIN AGGREGATE function such as DCount, DSum, DMin, DMax, DAvg, etc. The function will include a single field name, a domain (a table name or query name without the word FROM), and possibly a criteria (like a WHERE clause without the word WHERE). A domain aggregate function call, in this case, counts as an "ordinary" field with a name that you would have to assign if you are returning values to the query, though if instead you used it in a WHERE clause it would not have to be named like a field.
You would use one or the other in a given situation. It would be very rare to use both. You CAN use a domain aggregate inside an SQL query but it is NOT recommended due to inefficiency concerns, particularly for larger tables. If you actually used the Domain Aggregate function in a query, it should ALWAYS be visible in Design view or SQL view (but of course would be resolved to a value in Datasheet view). If you used the SQL Aggregate, it will also be visible in Design view in the grid area, and will also be visible in SQL view. Again, in datasheet view it will be a resolved SQL function that is a single value per record.