It's a bit more work than that, but it's worth it. This may seem complicated, but it will be worth it in the end, and it's simply once you do it a few times.

You have to go down to "Tools", then "Manual Vote" and you'll see the slide adjuster there. Then go to "Account Operations" further up and click on the comment that you want to upvote. It will "open up" and you'll see "Parent Permalink". Copy that and paste back in "Tools-Manual Vote" where it says "link". The "author" spot should automatically fill in...make sure you're upvoting the correct author.

Like I said, it seems complicated, but is so simple once you try it a few times. You'll be able to vote so many more times by using it. Let me know if you get stuck. I've been meaning to write a post explaining it, so now that is on my list of to-do's tomorrow :)