To successfully install Adobe Flash into Gentoo Linux I took the following steps.

1. Added the line www-plugins/adobe-flash to the file /etc/portage/package.keywords

2. Added the line www-plugins/adobe-flash nsplugin to the file /etc/portage/package.use

3. Added the line www-plugins/adobe-flash AdobeFlash-10 to the file /etc/portage/package.license

Steps 1 and 2 may not be necessary, number 1 because the package is masked due to licensing issues rather than more generic issues, and number 2 because the package is a precompiled binary. If you discover one of the steps is unnecessary, pleas feel free to post which steps you successfully omitted.