There are two ways. You can edit the ebuild file of each masked package which you wish to emerge, and remove the ~ marker from the KEYWORDS variable field. The ~ marks the package as unstable, so once it is removed portage thinks that the package is stable. You will also need to do this for any masked package which is a dependency for the package you wish to install.
Secondly, (and my preferred method) is to set the ACCEPT_KEYWORDS environmental variable before running emerge. If you set it to “~x86”, then emerge will accept unstable packages for the duration of your shell session. You can do this with a single command like so:
ACCEPT_KEYWORDS="~x86" emerge -p <masked-package-name>
The -p flag will pretend to emerge the package, and so give you a handy list of all the packages which would need to be installed. This allows you to check and see if any more hazardous packages would need to be installed for you to unmask this package.
If you have try a masked package, make sure to report on your experience at stable.gentoo.org so that problems can be resolved and good packages can get unmasked.