What is the name of your Table/Query?
DSum ( expression, domain, [criteria] )
DSum("Price", "TABLE/QUERY", "Article = Baby Milk")
http://msdn.microsoft.com/en-us/library/office/ff193998.aspx
Video, basic DSUM.
http://www.599cd.com/tips/access/130809-dsum/?key=AlexForum