Yes, that will be your problem, as the ability to add tasks via URL used by this bookmarklet is a 1.8 feature for the desktop software. 1.8 is in "sneaky peek" release, and not shown to you by the software updater until you have previously installed a 1.8 version because you have to make a conscious decision to use the alpha-test software. You can read more about the 1.8 sneaky peek releases, including what new features they incorporate, at
http://forums.omnigroup.com/showthread.php?t=15345. I do not recall any changes which would make it difficult to switch back to 1.7.5 if you try it and decide you don't like it at the moment.