As Roflcoptr says, your Thalys is booked to leave from Platform 5.
Your best bet for platform 5 is to exit the Eurostar platform on the exit about half way along the train - not the one at the very front of the train. This'll drop you down by the Thalys lounge, and pretty much by a set of stairs up to platform 5. There's also a lift there too, handy with your luggage.
You should be able to make the platform change in about 5 minutes, so as long as your Eurostar isn't late you should be just fine
If you look at this map of Brussels Midi station, if you take the mid-platfrom exit, that's the set of lifts/escalators in the blue circle in the bottom right. Just above that is marked Cybelys, which is the Thalys lounge, and the lifts + escalators just above that are the ones onto the Thalys platforms.
(If you were to take the exit at the front of the Eurostar, that drops you at the "Arrival Eurostar" on the map in the bottom left, then you have to walk all the way round to the red Thalys area to be able to get up to the platforms)
