How can I attach the divider for sliding closet doors to concrete?
Use plastic concrete/wall anchors with long pan head sheet metal screws. Pick the diameter whose screw matches the holes in the guide you are mounting. Use masking tape over the carpet and mark the hole locations using the guide you are mounting on the tape. Leave the tape in place while you drill the pilot holes. Drill through the carpet, pad and into the concrete. Make sure you drill deep enough in the concrete to completely sink the plastic insert. 

Leave the tape in place or you will lose the holes. Insert the plastic inserts. If the carpet and pad is thick,  be careful to push the insert into the hole as far as possible, then use a hammer over the hole to carefully seat the plastic anchors fully. They should be fully in the concrete. Place the door retainer over the tape and start the screws. Tighten the screws,  drawing the retainer against the floor. Once in place,  you can trim around the door retainer and remove any visible tape.