Does your MX64 have a public IP address on it? If not, and it is sitting behind something else doing NAT you'll need to forward udp/500 and udp/4500 to the MX.
Are you sure the PSK is the same on the client as configured on the MX?
The next possibility is you might be having problems with the client's NAT device. I have a wizard that can create the client VPN configuration and it sets all of the settings to make it work properly. You might like to give it a go.
While attempting to connect, try taking a packet capture on the WAN interface of the MX (Network-Wide > Packet Capture) and confirm that traffic from the public IP of your client computer is reaching the MX. If it doesn't it would indicate an issue upstream or on the client device.
This KB article also gives some great troubleshooting steps: