I got my answer so I thought I would post it in case there were others with similar questions. (I noticed there were several views to this posting but no replies so maybe it's not just me who was confused). Anyways, here's the email I received from the Belgian Consulate office in Washington DC.

There is no visa for American citizens to go to Belgium to get married.

You need to get married before the period of 90 days is over.

Please ask your Belgian fiancé to contact the commune in which you want to get married so they can let him know which documents are needed for you to get married in Belgium. You will need to take care of these while you are still in the US.

As soon as you are married in Belgium, you will be able to apply for a residence permit.