Look up the quest on YouTube their always a walk-through. Always watch the questing on YouTube the reading it it faster easier. You will thank me later. Just look up a video on which questing your doing copy the video and you done !
If you need any help in the Runescape questline you can always do /wiki <questname>.
This works for anything, so you can do /wiki man and it will open the runescape wikipedia for you.
More explanation on the issue would help otherwise using a youtube video may be better than anything we can suggest. If I were to guess it could just be that you're not actually "use"ing the key on the trapdoor or the other items on the appropriate places.
Sometimes just clicking "open" isn't good enough so try to right click -> use and then use it on the appropriate space.