Travel by train
If you travel by train, then be careful when you buy your ticket: you want to get to "Louvain-la-Neuve Université", not to "Louvain" or "Leuven". Louvain and Louvain-la-Neuve are two different cities. Use the following site to get the connections: https://www.belgiantrain.be/en/home To get here from the airport type in From: Brussels Airport and To: Louvain-la-Neuve.
The train station is two floors below the arrival hall. It takes a little more than an hour, usually with a change at Brussels Nord and Ottignies, though there are options with one or even no stops, depending on time. A taxi will cost you around 100 Euro (33 km, about 25 minutes).
Those arriving to the Brussels South station can take any train going south; nearly all of them stop in Ottignies. It takes 6 minutes by train from Ottignies to LLN.
Martin’s hotel is located 5 minutes walk from the LLN station. The town centre is pedestrian only, with all the car traffic underground, so your GPS may be misleading. Here is the map, the station is on the right, hotel – on the left:
One can come to the hotel by car, there is a parking underground. Keep in mind that the roads are underground, thus your GPS connection will be lost. The address of the hotel is Rue de l'Hocaille 1, 1348 Ottignies-Louvain-la-Neuve.