I installed it by adding manually the repository. You just need to enter data from the text file you got by clicking the link at maemo.org:

Go to “Tools > Application catalogue” and click “New”. Then enter:

"Catalogue Name" field can be anything, e. g. "Mollux.org catalogue"
"Web Address" field should be "http://www.mollux.org/maemo/"
"Distribution" field should be "chinook" (or "bora", if you use OS2007)
"Components" field should be "user"

Click OK, refresh the app list and then install tagtool as usual, see e. g. nokia-n800-finding-and-installing-new.html