They may not be showing to you if you're signed in, because they don't want to charge you for an impression to yourself.
Also, depending on how many days have passed and the subreddit you've chosen, 800 impressions may be a very small number of overall pageviews, in which case only a small percentage of people will see the ad.
I eventually saw the ad while I was logged in. It appears that ads are only shown for a percentage of page impressions, so even if I purchase all of the available impressions, my ad won't be shown for all pageviews. I suppose this makes sense, I just wish they made it more clear.
Also, depending on how many days have passed and the subreddit you've chosen, 800 impressions may be a very small number of overall pageviews, in which case only a small percentage of people will see the ad.