DA1- Go to the Pearl. A fight should break out in the main room and all you have to do is wait til the fight is done. After the fight, you should be able to talk to her.
DA2-You don't get a quest for it but you can get her after meeting after getting all the party members (Except Sebastian). All you have to do is go into the Hanged Man and a cutscene will play. I don't think it matters what time it is but if it doesn't work during day, try at night.go to the hangman and she will be there hope this helps go to the hanged man after recruiting anders
Isabela only shows up in act 1, in the hanged man after aquiring Anders and his maps for Varric. If you miss her before the end of act 1, then you don't get her on that playthrough. A shame too, she's quite the character.