OS: Mac OS 10.6 (Snow Leopard)
Connecting to your Juniper Networks device via web browser and clicking "Start" under the "Client Application Sessions" section to kickoff a VPN session causes the browser to seem to attempt to install the Network Connect client, but after a short time it simply returns you back to the main menu page. No VPN session is established.
It seems there is some compatability issue with Mac OS 10.6 that is causing the problem.
Check to see if you even have a "/Applications/Network Connect.app".
If not, download and install from:
Then in a shell:
sudo chmod 755 /usr/local/juniper/nc/6*
sudo mkdir '/Applications/Network Connect.app/Contents/Frameworks'
Logout of the Juniper VPN web portal and log back in. The Network Connect app should start automagically - if not try clicking "Start" again.
You should now have a VPN session!
Hope this helps someone out there.