Hi,
I´ve put up an ad on right sidebar (on single post pages) at www.sudurnes.net. The ad does not show up however the title shows (x) I´ve gone through everything and can´t seem to fin the problem.
I took a look at your account and your site ("View Source"). It looks like the AdPlugg SDK has been added twice but that you don't have any ad tags. The no-ad-tags issue usually means that you haven't added the AdPlugg Widget. It may be easiest to get a simple single ad working (use this tutorial) and then add more complexity with Zones, Placements etc after.
Also, if you use Placements, make sure that all enabled ads target at least one Placement. Ads that don't target anything specifically, go everywhere. Please message us using our contact form for more help specific to your account/site.