howto write permission

[Paul <paula4l@access4less.net>]

I wanted to give my self write permission to /usr/local, so did 'adduser 
paul staff', which succeeded.

paul :~$ cat /etc/group |grep staff
staff:x:50:paul

Also, debian sets up /usr/local to be rwx for group staff:

paul :~$ ls -l /usr/
total 150
drwxr-xr-x    2 root root  58080 2005-01-29 23:04 bin
drwxr-xr-x    2 root root    504 2005-01-23 20:06 doc
drwxr-xr-x    2 root root   1392 2005-01-23 09:36 games
drwxr-xr-x   50 root root   6136 2005-01-26 20:08 include
lrwxrwxrwx    1 root root     10 2005-01-17 21:20 info -> share/info
drwxr-xr-x  136 root root  71648 2005-01-29 23:04 lib
drwxrwsr-x   12 root staff   336 2005-01-27 15:49 local
drwxr-xr-x    2 root root   6280 2005-01-29 23:04 sbin
drwxr-xr-x  238 root root   6608 2005-01-29 23:04 share
drwxrwsr-x    7 root src     280 2005-01-28 19:46 src
drwxr-xr-x    6 root root    144 2005-01-20 12:29 X11R6

showing that staff has rwx privelege for /usr/local. However, when I try 
to make a directory, I get:

paul :~$ cd /usr/local
paul :/usr/local$ mkdir testdir
mkdir: cannot create directory `testdir': Permission denied

What am I doing wrong?

paul