Steps 1 to 10 are for Servers 1.7.5 and below. For Servers 1.7.6 and above, Please scroll down and use steps 1a to 7a.
1.7.5 and below
1. Click files on your control panel
2. Click Config files
3. Click server settings
4. Choose enable whitelisting via the drop down arrow
5. Click save
6. Then click back
7. Click on Whitelisted Players (in the Config files section)
8. Add your ingame name and any players you want whitelisted (each on a separate line) to the box
9. Click save
10. Restart your server
1.7.6 and above
1a. Click files on your control panel
2a. Click Config files
3a. Click server settings
4a. Choose enable whitelisting via the drop down arrow
5a. Click save
6a. You can add the player you wish from the console by typing: whitelist add playername
7a. Then to reload the whitelist, type: whitelist reload.
If one of your players has issues connecting after this. On your control panel, click players, find their name, click it. Change their access to guest.