Hi Romain
Thanks for using Agile Multiple Seller/Vendor module as your prestashop based online market solutions.
Can you give us a little more information? Other 3rd party modules installed? Do you get any error messages, or what happens when you try to assign a seller?
This might caused by incorrect installation or configuration of your store. Please check it for more details.
The module need modify informaitons.tpl file and add list_header.tpl file during installation. This is done automatically by the module during installaiton. If for some reason (most cases is permission) this is not done correctly, then the issues you reported will happens.
Or if your store configured the Smarty to "use cache" and "never compile", then even the files installed correctly, it will not be in effect.
See here for more details on the causes and solutions
viewtopic.php?f=9&t=1924