I'm not sure how you guys are interpreting this but ill provide an example just in case: Say there is an Anubis and an immortal Otyugh on the field. The Silent Siren would not be able to target the Otyugh since it has immortality. If it were to target the Anubis however, it would
not steal its ability (the creature keeps its ability), but you would be able to use it on that turn as if it were your creature. So you could force the Anubis to use its ability and cast immortality on
your Silent Siren, which would then remain immortal. The affected creature will not attack for you however, and your opponent will be able to use his creature on his turn normally. I am considering the possibility that any quanta cost of the borrowed ability would be deducted from your opponent instead of yourself.
And Flayne, read through ftbhrygvn's post in
http://elementscommunity.org/forum/index.php/topic,2632.20.html , it provides a good step by step instruction on adding everything you need to the card.