I agree with Topshelf. The obvious motivator here is the thing she wants to sniff. She probably couldn't care less about a treat at a time like this. So, try preventing her from pulling to the place where she wants to go sniff in the first place. Hold her snug and tell her "leave it" if she knows that trick. Walk around a little and when she is walking nicely, give her a release word which means she is free to go sniff. That is her reward for walking nicely with you. Then after a little fun of sniffing, "let's go." And you'll have to then use a high value treat to reward her for coming along with you. It's going to be hard to beat the fun of sniffing, so make it one heck of a good treat.
What breed is she again? Whatever is her "obsession," the thing she loves to do, she needs an outlet for. You can not supress certain drives that dogs have or you'll make them neurotic and unhappy. However, an outlet when and where it works for you and her is the best way. You, being her leader control this thing she loves. She needs to perform what you want first and then she gets to do that. Also, keep working on her basic obedience. That will help. Make it all positive, fun and games and she'll enjoy working with you and that drive she has will be put to good use. You don't want her to lose her drive, but you do want her to lose the behavior of her choosing when and where she's going to drag you over to smell something. You can eliminate that rotten behavior without ruining her drive.
You might enjoy getting her into tracking. Learn about it. It's fun and she may be really good at it. When it's in a controlled environment, what better way to let her use her talents and drives? Good luck.
