Essentially, exit the platform between Eurostar coaches 11 and 16. This 'quick connections' exit takes you to the ground floor area where there are lifts to the Thalys platforms. Just turn left out of the double doors and follow the signs to the Thalys trains, you'll recognise the logo! All you have to do then is take the lift to the correct Thalys platforms, check the departure board to see which yours is going from.
There you go, no need to worry now!
On the other hand, if you have time to waste at Brussels Midi there are plenty of shops and cafes to keep you busy whilst you wait for your connection. Just follow the platform down to the front of the train and head down into the main station area.
