The generic way to install applications on the N800/770 is to add repositories to the existing list of repositories in the Application Manager, and install from the list of applications you find there. The reason to add repos is to get access to even more applications.

The second-best *) way (when the app. in question isn't available in a repo) is to use the tablet to access e.g. maemo.org and find those one-click installation icons.

The third-best way is to find the .deb in question, download it, and use the application manager to install from file.

The fourth best way.. well, let's not go there yet.

And kismet can be installed from a repo, unfortunately I don't remember the URL but it can be found by searching the forum. (There are too many repos for the N800 and 770, and obviously anyone can add repos as much as they want, but this is one area where a future concolidation of repos will improve and ease the situation).

EDIT: *) In some ways the one-click way is the best way, as it's sometimes the simplest way to get the correct repository added to your device.