Earlier I showed steps to add Reminder add-in on earlier version of Ubuntu (
http://gusantov.blogspot.com/2008/07/event-reminder.html). This note is about how to add Reminder add-in on Ubuntu 10.04 (Lucid Lynx). I wrote this because there's slightly different in the way the add-in files of Tomboy are stored. In earlier versions of Ubuntu (correct me if I'm wrong), add-in files are stored in ~/tomboy/addins. In Ubuntu 10.04 these files are stored in /usr/lib/tomboy/addins.
So, if you want to add Reminder add-in, after you download the dll file (I choose the binary one) (http://flukkost.nu/blog/tomboy-reminder/), then move it to /usr/lib/tomboy/addins. You need to enable it (see my previous note) afterward. If you can't see Reminder add-in in there, you need to restart your system (this is what I did to make it appear on the add-ins list).
Good luck.