in the server.properties file check and tell me what your "server-ip=" is, wait.... not trying to get your ip, but if your using the wrong ip it sometimes comes back with a port being used error. If your just playing LAN you want to use your local ip example. 192.168.1.1 vs using your Internet ip. If you aren't playing LAN, you have to use your internet ip, it can be found by googling "whats my ip", then you need to port forward your minecraft port. DONT post your real IP address here. (To find out your local ip address, click "start" and type "cmd" it will bring up a dos command prompt, then type "ipconfig")
Might help a little, its for vanilla but setting up tekkit is very similar.