To install extension in openoffice first go to (click on image below to go to site) Extension Section of Open Office Website.
Download the extension you want by searching on that website and copy it to any folder which you will not be deleting.
No open any one of the openoffice.org applications.
Go to Tools—> Extension Manager as shown in figure
Now click on add in the extension manager window.
Now select the downloaded extension file and click on open.
Now you can see you extension installed. Check for versions for which the extensions are valid before downloading