To allow clients to connect to your VPN server certain ports need to be open to the Internet. If you have any sort of firewall such as a router, or other software firewall including Mac OS X's built in firewall you will need to specifically tell the firewall to accept incoming connections on these ports.
For PPTP connections, TCP port 1723 needs to be opened.
For L2TP connections, UDP ports 1701, 4500 and 500 need to be opened.
So, on your router, tell it to forward the relevant ports to the IP address of your computer running iVPN. Also, some routers have an option to allow a VPN pass-through. If your router has this functionality, make sure you enable the relevant pass-through.
In some cases, you will have to enable the appropriate VPN pass-through on the client-side's router also, otherwise negotiation will fail or hang on the client.
For specific help on forwarding ports on your router, refer to your routers instruction manual.