It's actually a bug in Mod Wordlets. I'll include a text file that has a working version of it in a few minutes...
Anyway, ZIP file updated. Rehack ubb_forum_summary.cgi and vars_wordlets_mods.cgi (the chat_visitors_X parts), and upload the updated ubb_chat.cgi.
Fixed: now it uses &IPAddress instead of $ENV{REMOTE_HTTP}.
Updated: now there is 1 wordlet for 1 visitor, more than 1 and less than 1.
Use %%URL%% to open a href, %%ENDURL%% to close it and %%NUMBER%% to show the number of visitors.