I also have some general information on installing programs on the Psion (I installed all my programs via Psiwin):
How to Install
If your Psion is connected to a Windows computer via EpocConnect or PsiWin 2.1 and later, you can usually just double click a .sis file. The program will be installed over the serial link.
If you transfer the .sis file to a Psion via IrDA or Compact Flash, or download it from the internet using a Psion, you can just double tap the .sis file in the Psion. Or you can open the Control Panel, and use the Add New option.
A few programs install better direct on the Psion. If you ever have problems starting a program installed via PsiWin, transfer the .sis file to the Psion instead, and run it on the Psion.
Residual .sis file stubs are created in the system\install folder during installation. They provide information for the control panel list of installed programs, and information for safely removing applications. Do not delete them manually.
If a program fails to fully install, it will be marked /partial/ in the control panel list of installed applications. Removal and reinstall is recommended.
Older programs may have to be added manually if no .sis file is provided. Follow the author's directions. If there are no instructions, try making a new folder in the System\Apps directory under the name of the program, and put all the program files in that. Data files should however not be kept in the system folders.
You can also download the Psion 5mx manual (click here)
Hope this helps!