[Typo3] ext manager issue on localhost

[mario]

hello,

when I try to import ext from the online repository, I get the following
error msg:

URL file-access is disabled in the server configuration in
/..../typo3/htdocs/testsite/typo3/t3lib/class.t3lib_div.php on line 1993

I guess is something I have to fix in http.conf and/or htaccess, but I
don`t know as much as to try.  
(note I am on my machine, on localhost. No similar problem on my ISP
machine) 

Any quick step by step help?
Thanks a lot
mario