You have to install it as it is not included by default.

It is located here:
http://downloads.maemo.org/product/osso-xterm-advanced

Clicking the green arrow should install the right catalog and install the app.

For some apps, I had to try a few times and save the install file and load it up through the application manager. I think that is a problem with malformed .install files as some have worked perfectly. I can't remember which category this one falls into. Hopefully it works for you.