Can you provide a screenshot?
If I understand correctly, you're unable to see the HDR dialog box in Photoshop. This may be due to various reasons.
Has it worked fine before? If so, it's probably a corrupt plist file. Follow instructions here to reset preferences (remanimg preferences bit you mentioned in your question): http://forums.adobe.com/thread/375776
If it has never showed up fine for you, your screen dimension is probably low in height. Photoshop calculates the display bound of your screen dimension when you launch it and displays your workspace accordingly - all panels & tabs flow into it. For some dialog boxes within Photoshop, they require a good amount of height to be displayed fully. This may be a possible cause why the HDR dialog is getting chopped at the bottom.
However, to be able to answer your question more precisely, as Julia had mentioned, please provide us a screenshot. Use the small camera icon here when you're replying to this post to attach it to the forum. Don't use e-mail - attachments via e-mail dont reach the forum.
On a Mac, Cmd + Shift + 3 takes a screencapture and saves as .png file on your desktop. On Windows, hit PrtScr (Print Screen) to copy screen contents, open Paint or something, Paste there and save the file as .jpg or .png to upload it here.
Hi. You’ve posted your question in a forum that is for beginners trying to learn the basics of Photoshop. I'm moving your question to the Photoshop General Discussion forum for specialized attention to your situation.