There are two airports in Brussels. If you come from Zaventem airport (the national airport of Belgium), you can catch a train to Maastricht. First you should get a train from the airport to Brussels North station, which leaves every 10 to 15 minutes. From Brussels North you can catch a train to Liege Guillemins (central station) and then change trains for Maastricht.
If you fly to Brussels South Charleroi Airport then you can also get the train to Liege (Guillemins) and then to Maastricht. You will have to take the shuttle bus A from Charleroi airport to Charleroi train station first though. This shuttle bus costs €5.
If you are travelling from Belgium to Maastricht, you should definitely check out the Go Pass 1 Maastricht! This is a special ticket which allows you to travel between Maastricht and any station in Belgium for only €7.50. However, be aware that you may have to pay a bit extra in order to get to the airport stations. You can order you Go Pass online here.