Imperative for 2nd person singular is "jedz!", for 2nd person plural "jedzcie!"
You can add z- at the beginning to make it perfective and therefore convey that the meal should be eaten whole.
Why isn't there a mark above the "s" in posilek? The pronunciation sounds like "sh".
It's not sz (sh), it's si (= ś). Many learners have a problems even perceiving the difference, but for Polish people those are completely different sounds. This is palatalized "s".