I assume that you need help formulating the WHERE clause of your query to allow for NULL conditions?
There are several ways to accomplish this, the best being to write your query as a stored procedure, either containing variable sections, or using dynamic SQL.
There are also ways to build dynamic queries in Reporting Services, but I don't have much experience with this.
Alternatively, you might try something along the line of the following syntax:
WHERE Account_Number = (CASE WHEN @AccountNumber IS NULL
THEN Account_Number
ELSE @AccountNumber END)