If Pure Calendar is not visible on your phone : you have to contact Google.
Each ROM/phone model is registered in Google’s servers (we speak about ‘fingerprint’). If a phone ROM is not reconized from Market application and servers, Google copy protected applications (like ‘Pure calendar’, but also a lot of other applications) are not visible from the phone. All my other applications are not copy protected by Google, this is the reason why you can see them.
Each new official ROM always needs few days to be registered.
So… either you contact Google … either you contact the handset maker… either you wait for ROM registration (generally few days, sometimes 2 weeks).
For custom ROM, you need to contact the ROM cooker to let him know the issue.
If you don’t find a way to fix this, you won’t see a lot of market apps (all copy protected apps).
I am sorry, but I can’t do anything about that problem…
EDIT 07/11/2010: You can try the ‘market enabler’ app available here, to fake some phone settings, but I am not sure if it works (please report your result).
EDIT 13/03/2010: Here are solutions for advanced users (root)
And you can search on Google how to change your fingerprint for your specific device.
EDIT 20/01/2010: You can now get all Pure widgets on AndroidPit alternative market