D-Link DFL-1100 User Manual page 97

Under MPPE encryption only None should be checked
Check Use IPsec encryption
Enter key 1234567890 (Note! You should use a key that is hard to guess)
Retype key 1234567890
Click Apply
3. Setup policies for the new tunnel, Firewall->Policy:
Click Global policy parameters
Enable Allow all VPN traffic: internal->VPN, VPN->internal and VPN->VPN
Click Apply
4. Click Activate and wait for the firewall to restart