I'm trying to get with
the meta_key where the meta_value is
But I do not get the value. What should I do?
I agree with @Simon Blackbourn, but for arguments sake, here's one technique:
Bearing in mind I haven't seen the rest of your code or know the context of your question, I would suggest that you're doing it wrong: you should always know the name of a usermeta that you're trying to retrieve.
If you're trying to determine if a post has been read by a user, you could save the user ID as a postmeta, maybe that's a better solution?