By default they have conflicting GUI IDs. You need to change the IDs of one or both in their configs, and make sure all your clients have the new configs.
By coincidence, I had that problem just before you posted it - nobody on our server was using the forcefields yet, so we just deleted that addon because it's easier.
OK thank you!
Do you have Modular Forcefields?
Yes. It is incompatible?
Could you upload version 3.1, because i want to install it on my bukkit server. On 3.1.1 the gui don't work. 
Thanks for reading 
Name:
Digestionator
Description:
The Digestionator is a machine, which is able to convert food into ICs. In the inside is an artificial digestive system, which is the main part of this machine. It is used
similar to a Generator, but much slower. You put in some food, where you normally put in some coal. EUs production=Hunger stilling level of the food (0-20)*10
Recipe:
The tin cans are empty and the zombie is rotten flesh.
