With the Mac German keyboard layout, you type @ using Option-l (lowercase L). Within CrossOver, we have a dilemma about what to do with the Option key. On the one hand, you might want to use it to access additional characters like @. On the other hand, you might want to send the Alt keypress to the Windows program. CrossOver provides a preference setting so you can decide how the Option key should behave. On Macs with two Option keys, you can configure each key independently. By default, CrossOver sends Alt to the Windows program when you press the left Option key and accesses additional characters using the right Option key (so it behaves like the Alt.Gr. key that's common on some non-U.S. keyboards).
So, you should be able to type @ using right-Option-l. Or, you can change the preferences to work however you prefer.