I copied this from the manual
"To remove the bench from the rails, the release lever should be held.
To make this work easier, a release mechanism can be accessed on the side of the bench:
You can insert a screwdriver to hold the release. You can then let go of the lever and push the bench to the furthest position forward and remove it."
Going by the picture you insert a long thin screwdriver into the hole while holding the lever to release.