There are several ways to get from Brussels Central Station to Jam Hotel.
Line 92 tram (about 19 minutes)
Just walk about 5 minutes from the Brussels central station heading to Palais.
Then every 15 minutes tram 92 will take you to Janson. Jam hotel is a 1-minute walk.
Cost: 3 Euro.
Source: Rome2Rio
The hotel is also walkable from the Central train station about 30 minutes walk (2.4 kilometers).