You won't be able to post a weight tracker until you have been here for 30 days and have 30 posts. We've had a lot of problems with spammers, so unfortunately we had to disable anything in the signature area for that long.
After 30 days and 30 posts you go into your user CP and choose Edit Signature. If you already have one set up, you go past the preview to just below where you'll see code. You likely need to scroll the code to the right until you see your weights. If they're all the same, you change the last one (your current weight) and then save it. If you choose preview at the bottom of the page it will refresh and you should be able to see the changes. Don't forget to save it after that.