Diksmuide has a train station with direct connections to Bruges (about 40 minutes) and Ghent (about 1 hour 20 minutes), via Lichtervelde. From Brussels, change at Ghent or Bruges. Trains run roughly hourly. The Trench of Death and IJzer Tower are about 2 km north of the station; a taxi or a short bike ride will get you there, or it is walkable in 25 minutes.
By car, Diksmuide is just off the A18/E40 motorway linking Bruges to the coast. From Bruges it is about 30 minutes; from Calais via Dunkirk just over an hour.