there's something bugging whenever you log in somewhere where someone's built something new, causes people to crash when they log in. make a room somewhere with a sign saying 'please don't build here' and try to log out in there
i've been running down to an empty space and /tp'ing people to myself if i see them having login issues and that seems to fix the problem on the next login attempt, if you log in and you're in some weird place that's probably why.
You need to grab the Tekkit launcher, select "TEKKIT" in the (graphical) top left dropdown menu, then click options and select "development builds". then login as you normally would with minecraft, add the server to your list, and you're all done.