Just go to the DB website.

Select "return journey". Insert "Frankfurt(Main)Hbf" in the from field (assuming you want to depart from the city center), insert "Bruxelles-Midi" in the to field. Select the date and time of your journey.

Now you will get a list of connections. Select the one with zero changes, and marked "ICE" in the products column. Add the return. Then select "check all" in the savings fares columns. This will give you the price for a return. For the dates I checked, 88 Euros, but it could be only 78 Euros (but not less).

You can then purchase this fare online. You pay 4 Euros extra for a seat reservation, but this is not necessarily needed depending on the time.

You should then select to buy an online ticket. This is a ticket you print yourself, which has a barcode that is scanned on the train. You need to also have the credit card you used for booking with you when you take the train!

There are 3 ICE direct highspeed trains a day from Frankfurt to Brussels, and they are pretty comfortable. You can get seats directly behind the driver and see the landscape passing by with 300 kph (one direction first class, the other direction second class). Not possible to be sure to have a reservation for these seats online.

And finally, this thread should be in the European Rail forum