it seems from the looks of things that you don't actually have extras-devel enabled, however wrt-api exists there only for now..

so either enable extras-devel and then update the repositories and try again (easiest, recommended)

or download the package manually to your n900 from here

http://maemo.org/packages/package_in...wrt-api/1.1.0/

either using the browser, then using application manager to install

or do the following

cd /home/user/MyDocs/
wget http://repository.maemo.org/extras-devel/pool/fremantle/free/w/wrt/wrt-api_1.1.0_armel.deb
sudo gainroot
dpkg -i ./wrt-api_1.1.0_armel.deb
apt-get install openssh-client openssh-server

in case you get errors about unmet dependencies you will need to download them as well, to get a package directly, search google with the following :
maemo package <packagename>

so my suggestion is instead of all this, simply enable extras-devel repository, update the repos, install openssh as needed then disable extras-devel again if you want (recommended).