Why are you doing it that way anyhow?
It's wrong, for at least two reasons:
1. You should use installpkg(8) to install packages.
2. If you elect to extract the package contents manually, you would need to do it something like this:
Code:
tar xzf package.tgz -C /
cd /
sh install/doinst.sh
Note that doing it the second way is dumb, but it's your system.