For installing user extensions, just follow the below steps:
1. Go to http://www.google.com.
2. Type selenium documentation in search box and click on the first link.
3. On selenium documentation page, click on "Sequence of evaluation and flow control" link . In the description of this link,click on " goto_sel_ide.js extension" link. Then click on "page" link. One blog will get open. Scroll down and click on " github.com/darrenderidder/sideflow " link and then download the zip file.
4. After extracting that file, you will get one .js file. Ggo to Selenium-IDE’s Options=>Options=>General tab. Browse for that file and press ok.
5. Now close Selenium IDE and restart it.
For checking whether its added or not, type while in command box and see it will start displaying commands.
Your user extension is added now.. :-)