Prompted for VNC password in mac OSX 10.5.8

 Prompted for VNC password in mac OSX 10.5.8

When i try to connect to my imac throuh my iphone 4g OS 4.2.1 it tells me:

Error: VNC: authentication failed: 
Incompatible Version. Version.

If you are connecting to a Mac, we 
recommend you install the 
PocketCloud Mac Companion.
Alternatively, make sure you enable 
VNC Viewers and set a password in 
the Remote Managment Settings.

I tried to set the password in the Remote Acces (VNC) Setup menu that appears in the Mac Companion but i keep getting: 

VNC password is not set. Prompt for VNC access password.

What do i do?