That depends on which form of the forum that you are using. If you are using the xda app, then you have to tap near the top of the post that you want to thank, and it will be knew of the options.
If you are using the Tapatalk app, there will be a thumbs up in the top left corner of the screen.
If you are using a browser in desktop mode, or an actual PC, the thanks button is on the bottom of the post, towards the left. Make sure that you are signed into xda, and not browsing as a guest.
Just an extra tidbit of info: there are no thanks button within the off-topic section of xda.