The plugin's release is hosted on Selenium HQ here
Once installed you should find the following menu options in Selenium IDE:
- File -> Export Test Case As... -> Grails Selenium RC (JUnit 3)
- File -> Export Test Case As... -> Grails Selenium RC (JUnit 4)
JUnit 4 tests are appropriate if you are using Grails 1.3 or above. The exported test case will not extend a base class, and will use assertThat along with Hamcrest matchers rather than using assertEquals, etc.