The EpocYacas-*.zip file is a version of Yacas that runs on a Psion
series 5 or compatible.

In the zip file there is a stdlib.sis file which you need to install (if it 
doesn't auto-install when dragged to the device, just tap on it on the 
device itself).

The other three are Yacas.app, Yacas.rsc and a directory scripts\ . You need to 
put these three in the directory \System\Apps\Yacas\ (create the directory Yacas).

Enjoy!
Ayal