What should I do if my advisor is forcing me to vote on a specific candidate in IEEE elections?
Allow me to hypothesize a different tack from most of the other answers: Simply don't engage in this issue at all. As my mentor said once, "You constantly get institutional demands for some action, in some cases it's best to ignore them, and you'll find that many just go away." 

This request is so incredibly unethical, and also so completely orthogonal to your responsibilities as a graduate student, that it's hard for me to imagine your advisor spending time on tracking you down and taking revenge over it. If you think you might receive more pressure later on (e.g., I've sadly cracked under such circumstances), go ahead and cast the online vote in the privacy of your own home, and then later on say, "Oh sorry, I got a notification and cast the vote immediately." 

The request is so inflammatory that I don't see any use to "politely having an open discussion with your advisor". I think that only opens the door for defensiveness and retaliation. As wildly unacceptable as it is, I don't see it as being a good use of your time tracking down where to lodge a complaint over this matter (which your advisor seems canny enough to avoid documenting except verbally, so it would be just your word against his, according to comments above). 

At least consider the null action as a possibility.