How Do I Use {$excerpt:n} ?

Thank you for this plugin.

I have been trying to use the code {$excerpt:n} to limit my post to 50 words, but is showing the code back to me on the past page, i tried using it as {$excerpt:n=”50″}, still did not achieve having 50words.

Please help!.

Thank you.

https://ww.wp.xz.cn/plugins/rss-post-importer/