You may need to "downgrade" your client back to 1.7.5. (The server can run 1.7.2-5)
Dont worry, this is really simple. On the main launcher, before you log in, click on the "edit profile" in the bottom left hand corner. In this image, it is slightly blue and under "main" (This is my personal thing)
http://i.imgur.com/CiJPeAR.pngThis will bring up a pop up. In the second "box", click the drop down that says "use latest version" and click "release 1.7.5". This will set it up so that it will download 1.7.5 and use that version, and hence go on the server
http://i.imgur.com/AHHkOz1.pngWhen the server/bukkit updates to the new 1.7.6 minecraft edition, you will need to follow a similar process in order to update to 1.7.6.
Hope this helps!