"Der Bueger ist im Verein"
I'm detecting dative here with that "im" for "in dem". Warum?
You're right - it's a dative. Dative and accusative are not only used for objects, but also after prepositions. Certain prepositions trigger certain cases."In" is a so-called "two-way preposition", which means that it can be followed by either the dative case or the accusative case. The accusative case is used when you're talking about movement ("Ich gehe in das Haus"/ I go into the house) and the dative case is used when you're talking about a fixed location ("Ich bin im Haus/ I am in the house). In your sentence, there is no movement and thus the dative case is used after "in". See: http://coerll.utexas.edu/gg/gr/cas_09.html