1. Open user-extensions.js with an editor (Notepad, etc.); it's found in the seleniumcorescripts folder. If it doesn't exist, just create it.

2. If you need to, commit the user-extensions.js file (like if you use subversion).

3. Open Selenium IDE and choose the Options menu and then Options... from that menu.

4. Make sure the path to your user-extensions.js file is entered in the Selenium Core extensions field (like seleniumcorescriptsuser-extensions.js)

5. Press OK button on options

6. Restart the IDE to reflect your extensions.

Note: After reopen, Selenium IDE may show compilations errors if the user-extensions.js file has any syntax errors.