Venus is a shell which is compatible with ION.
A shell is a program used to launch others programs (bypass some limitation, squished programs).
To install Venus, download it here :
http://www.ticalc.org/archives/files/fileinfo/171/17170.html
Then transfer it to your calc. Go to PRGM menu, then run A program.
You see nothing ? This is because you haven't installed any programs which require Venus.
Venus is also compatible with some other shells like Ashell, SOS ...