I had this problem until I opened the launch.bat file and before the command that's already written there, I navigated to where my Tekkit.jar file was stored
Originally the file (launch.bat) looked like:
java -Xmx3G -Xms2G -jar Tekkit.jar nogui
pause
which I modified to:
cd C:\Users\Ben\Desktop\Minecraft\server
java -Xmx3G -Xms2G -jar Tekkit.jar nogui
pause
cd is the change directory command in Windows command line, obviously that directory is going to be different for any one else, so you just need to change that line to where your Tekkit.jar file is
Hope that helps others
