It can be downloaded from the extension’s page or from the GitHub release page. If downloaded from GitHub, make sure to place it in the /ext/phpbbservices/selectivemassemails directory.
If you administer a phpBB board, you should find my book Mastering phpBB Administrationinvaluable. Learn lots of tips, tricks and secrets I learned from serving over 400 clients. A paper (7.5 x 9.68 inches) and an ePub version are available.