Install from update site
Select Help / Install New Software.
Enter the update site URL in "Work with:" field
Update site for release: https://testng.org/testng-eclipse-update-site
Make sure the check box next to URL is checked and click Next.
Eclipse will then guide you through the process.
You can also install older versions of the plug-ins here. Note that the URL's on this page are update sites as well, not direct download links.
Links
https://testng.org/doc/download.html
