If your on a browser there should be a star symbol next to the downvote button, but if your on an app it might be different.
If it isn’t working then it’s probably just server issues or an issue with the app. Try pressing the save button once waiting a few seconds then refreshing the page if you want to check to make sure it saved.
I don’t believe you can see Mastodon posts from Lemmy unless a Mastodon user comments on a Lemmy post.