How to open a LAN-World

[Pottercraft]

Hello everyone! I have tekkit for like 2 months now and i wanted to play with my sister on my own singleplayer world. This is what I did and what happened:

- I did ESC

- I clicked on Open to LAN

- Then I saw Port: and IP:

- I didn't know what that was so i clicked on start LAN-World.

- My sister couldn't join and then i saw in the chat: Error: Make sure you have a correct IP entered.

What is this and how do I get a good IP?