That type of message, in general, means the server is expecting XYZ version of the mods listed. First thing you need to do is determine which side needs to be updated, server or client. More than likely your client was updated and the server needs to be...
At your Minecraft screen click the Mods button. Search the list to find the mods required by the server. If you are like the rest of us it was:
Steve Carts 2.0.0.a41
ExtrabiomesXL 3.7.0
Forestry 1.6.4.1
SoulShards 1.2.1
RailCraft 6.12.20
EE3 pre2a1
PortalGun 1.4.6.2
So long as your client has the same or slightly newer versions you should be ok on the client side. Also note in the upper left corner of your minecraft window it should show: Direwolf20 Pack v3 or v4 (v4 is current).
Now, quit out of Minecraft and relaunch your FTB Launcher. Click the Filter Settings button, select Mod Pack Type: Server, click Apply Filter.
This will return you to the launcher and now the "launch" buttons should say... Download Server ModPack... get that file and upgrade your server files with it.
Good luck!