They never existed. The town Wolin existed, and is mentioned in contemporary sources. Adam of Bremen for instance wrote in great superlative terms about the town Wolin, which he called Jumne (Old Norse Jom). Yet neither he or any other contemporary had anything to say about any supposed Jomsborg castle or colony of Danish vikings. Did they exist?

To quote the historian Lauritz Weibull, who took them to task in part (Exkurs II - Jomsborg och jomsvikingarna) of his highly influential work Kritiska undersökningar i nordens historia omkring år 1000:

The answer cannot be in doubt. Jomsborg and the jomsvikings were unknown for 200 years after the time when they supposedly existed. Once they are belatedly brought into history, a constantly growing tradition attributes to them a series of feats that had earlier been attributed to others. At the same time two contradictory but interdependent theories on the founding of the borg and colony. Palnatoke is then inserted as one of the founders, and for this purpose is moved from Denmark to Venland and after several later metamorphoses becomes the chieftan of the warrior flock. The nordic Jomsborg and the jomsvikings that the Danish and Icelandic tradition relate, have never existed.

But although this is the case, it is also the case that some of the persons of which the jomsviking community is supposed to have been comprised, are historic to an extent. Among them the danish vikings about whose feats in the battle of Hjörungavåg was told by contemporary Icelandic skalds. At Jomsborg both Sigvalde Jarl, Bue and Vagn Åkesson meet. But these, as with the other jomsvikings, are lifted here onto a plane of existence outside of reality. The Jomsborg at which they reside is not a castle such that ever existed in northern Europe in the tenth century. It is the ideal castle, a giant sea castle, half jutting out into the sea, enclosing a harbor so large it could at once contain 300 longships; a large stone vault arched over the entry with a large citadel above it, and the opening below was locked from the inside with iron doors. The castle's chief Palnatoke has something of a Greek hero about him; he is the founder of the castle, he organizes the viking community, he gives its laws after the council of wise men. He himself wins victories practically everywhere; he is the most famous viking of his day and appears as the superior to all others in wisdom and sense. About the other Jomsvikings the story relates "it is generally said that one has never found greater heroes or fighters than the jomsvikings, and we believe that their like has hardly ever been seen."

(my translation)

It's a fairy tale.

Cool, thank you :D