I think the IN function requires parenthesis. So your syntax would be:
=If({Accounting Event Description} IN ("*- A-*"), "Works", "DNW")
That said, I don't know if the IN function support wildcards. Your best bet, if you're only looking to find one set of characters, is to go with Holger's suggestion and use SEARCH, but I think the syntax in Holger's example is backwards. It should be:
=If(Search("- A-",{Accounting Event Description})>0, "Works", "DNW")