If you're trying to follow lemmy's content from your mastodon account you should type "https://lemmy.ml/c/asklemmy" (without the quotes... and also omitting https:// should work as well, as well as typing "@asklemmy@lemmy.ml", again, without the quotes) into the search field on mastodon, and it should pop up there, and you should be able to follow it. Repeat for any other lemmy community you might be interested in.
You won't be able to post to the community, but you'll be able to read it, and reply to posts and comments in the community, from your mastodon account.