L2TP IPSec VPN connection gives better data protection than PPTP VPN connection, we therefore have decided to phase out the support of PPTP on Mac by 1 August 2008. If you have not yet switched to L2TP IPSec protocol, please make the move now by

1. Upgrade your OS to MacOS X 10.4 or above (as previous MacOS X version does not support L2TP IPSec VPN connection)
2. Follow the procedures published at
   1. Mac OS X 10.4
   2. Mac OS X 10.5

Reference:

Termination the support of PPTP VPN