Accessors are the software that allow Truesync Plus to access different PIMs, such as Outlook, Lotus Notes, Excite, Yahoo, etc. You load them from the TrueSync Component Install (TCI) a web-based program that you run from your PC.
However, you may already have the necessary accesors on your PC - I can't remember, but I'm pretty sure that Outlook accessors were loaded by default.
Unfortunately, I haven't used TrueSync Plus for many moons, and don't remember where in it's menus you set up with accessors you wanted to use. I think if you wander around a bit in the TS Plus menus you'll find something about accessors and will be able to go from there.
Also, go to www.rex.net (or look on Intel's site if the REX.NET site is down) and download Truesync Component Install and run it to get the most up-to-date versions of the TrueSync Plus software and accessors.
Hope that helps.
If you're still stuck, try posting a question on REX Rumor Central - REX 5000 users hang out there regularly.
Dana