Eurail reservations

First of all, before you buy your pass, make sure you have compared the cost of the pass, plus all the extra fees that get tacked on, to the cost of point-to-point tickets. Get the price of these from the national rail websites, not from RailEurope or Eurail. For Belgium: http://www.b-rail.be/main/E/
For the Netherlands: http://www.ns.nl/cs/Satellite/travellers

The passes sold by Eurail and RailEurope almost never make economic sense for travelers within the Benelux countries, because point-to-point ticket prices are pretty cheap. Reservations are neither needed nor possible.

For rail travel within Belgium, there is a pass sold by NMBS (Belgian rail) for about 70 euro that allows for 10 individual trips. The advantage here is that more than one person can use the pass at a time. So, for example, three people making a round trip from Brussels to Brugge count for 6 of the 10 trips. You can buy this pass at any train station in Belgium.

Depends on what trains you are using. Reservations are not included in the Eurail pass, so if you are taking a train with compulsory reservations, you will have to buy them separately. Unless you are taking a really popular train, you should be able to buy reservation when you get to the train station.

Like the above poster said, do some research (via bahn.de and Belgium/Netherlands/Luxembourg's national rail websites) to see if buying point-to-point might be cheaper. I don't know about Benelux, but in Germany, I found that point-to-point is almost always cheaper than a pass.

You said that you will arrive in Germany. The German Rail site, www.bahn.de, offers advance purchase Savings fares for some routes from Germany into neighboring countries. Book up to 92 days in advance.