Unzip the file.
$ tar -xvf BISP-.tar.gz
Install BankID
$ cd BISP-
sudo ./install..sh -i
The last part is only for 64-bit OS and that is to simulate your 32-bit Firefox pluging to run on your 64-bit OS. To do that you need the ia32-libs and nspluginwrapper packages. On a clean installed computer they are not installed.
To install them
$ sudo apt-get install ia32-libs nspluginwrapper
Then install the BankID plugin in Firefox.
$ sudo nspluginwrapper -i /usr/local/lib/personal/libplugins.so
Now restart Firefox and test your plugin via https://install.bankid.com/
To read more in depth of the installation, see http://ubuntu-se.org/wiki/NexusPersonal.