In make.conf:

ACCEPT_LICENSE="*"

To unmask all licenses. Or else name the licenses you wish unmasked one by one. In a recent portage-2.2 rc. It masks end user license agreements by default (-@EULA is the syntax)

Another option is to create /etc/portage/package.license and put the
licenses you want to accept in here. The portage man page has a little info
about it.

So if you see something like;

#
!!! All ebuilds that could satisfy "games-fps/doom3" have been masked.
#
!!! One of the following masked packages is required to complete your request:
#
- games-fps/doom3-1.3.1304 (masked by: package.mask, DOOM3 license(s))
#
/usr/portage/profiles/package.mask:
#
# Chris Gianelloni (3 Mar 2008)
#
# Masking due to security bug #194607 and security bug #204067
#

#
A copy of the 'DOOM3' license is located at '/usr/portage/licenses/DOOM3'.
#

echo "games-fps/doom3 DOOM3" >> /etc/portage/package.license