Steps 1 to 8 are for Servers 1.7.5 and below. For Servers 1.7.6 and above. Please scroll down and use step 1a
1.7.5 and below
1. Open your control panel
2. Click on your server
3. Click Files
4. Click Config Files
5. Click Operators
6. Add user name (one per line) in the box
7. Click Save
8. Restart your server
1.7.6 and above
1a. You can add from the multicraft control panel console using: op <playername>
Here is a link to some OP commands.