[TYPO3-english] Cannot add extensions using Typo3 4.6.8, why/how?

[Benoit Montereau]

I use to run Typ3 4.4.x and upgraded a first website to 4.6.8.
Unfortunately I cannot install any new extensions!

Using extensions manager -> import, the extensions is loaded (confirmation
is displayed) but NOT installed.

I just installed locally (xampp on windows, so no privilege problem) and
tried to do the same, still unable to get any new extensions.

Am I missing something here? In previous version, the extensions were
loaded and then we use to need to INSTALL them. Nothing similar in 4.6.8,
so how can I complete the install process?

Thanks y lot for you help to solve this very stupid situation.
Regards,
Ben