Hey,

Finally found a solution : by redownloading my enterprise package, I had to fill my license key in a form, hence opening a web session on odoo's portal.
When I activated a module on my local environment development : instead of displaying the "purchase a license" page, I think the system queried the portal with a valid license, and enabled the module locally.
In short : you need to be identified on the portal with your license to enable enterprise module on a local dev server.

(and as I don't have enough karma/xp I can't close this topic :-D)