You live and learn... I first thought that "marriage encounter" was just another name for a dating service but it turns out that these are (mainly church-organised?) weekend programmes attended by married couples, and are not meant to "repair" bad marriages but "make good marriages better", according to what I read on a few related sites. Here is the Wikipedia
Marriage Encounter entry.
As for Marriage Builders, I think the best explanation can be found here:
Marriage Builders