Why can't I use the full resolution of my UltraSharp U2713HM with my macbook pro retina?

I bought a Dell U2713HM because it was highly rated and had 2560x1440 resolution, but, after learning that the HDMI connection only provides 1980x1080, I purchased from Apple their Thunderbolt to DVI connector and tried that. Still 1980x1080. I've now ordered a mini-DisplayPort-to-DisplayPort connector and am praying that I get the full resolution that way, but I've seen a number of user group comments that you need a completely separate (over $100) Thunderbolt data (not display) interface to get the full resolution. If that's true, I'm returning the Dell. I've also seen people say you need to reset the PRAM and SMC settings, but I've tried doing that (I think it worked) but nothing changed.

HELP!

get a Mini DisplayPort / Thunderbolt to DisplayPort Cable $17.99 or less, thats what i did works fine, although photoshop still displays non retina so have to scale to 50% which is a pain.

I don't think an ordinary DVI connection would be enough to reach a 2560x1440 resolution. For that you'd need a dual-link DVI connection, and in your case a mini-DisplayPort to dual-link DVI converter like this one. It costs $99, but you could probably get a generic Chinese clone on DealExtreme or eBay for much less. I can't speak for the mini-DisplayPort to DisplayPort converter as I'm not familiar with the DisplayPort standard.

Today I made this happen on my Mac Book Pro 15" Retina Late 2013 model by simply using a HDMI cable. Apparently recent version of HDMI supports 2560x1440 resolution...

You then have to configure you monitor to Not do any magic picture improvements and set Sharpness to 0 to avoid shadows and unclear text.