I cannot really find a good explanation in English, as I think there isn't a corresponding word. The word "jo" is used to give a statement extra strength, like "det var jo det jeg sagde!", which means "that's what I told you!", with extra emphasis on 'told'.
'Jamen' is a "filler" word used mostly in spoken Danish. Frequently, when a person is asked a question, he or she - in order to win time to think - starts his/her answer with 'jamen'. 'Jamen' - directly translated - consists of two words put together: 'ja' (yes) and 'men' (but). However, it doesn't actually mean anything.
In written Danish, 'jamen' can be used in its actual meaning ("yes, but"): e.g.: "Jamen det var jo det jeg sagde!", meaning "(yes) but that's what I told you!" (wiht emphasis on 'told')