Maybe it depends on the computer? On mine (Mac, OS X 10.6.8), I have to plug in the Nook after launching ADE 2.0. I'm also not sure if it's required but might want to verify (1) your computer sees it and (2), when plugged in, the Nook's already set to MTP (Media device) (one way to check: select the time > "All Settings" below > "Storage Management" on the left > "USB Connectivity" top of second column > first option). I just checked with the camera mode and the software's not seeing it. If your computer doesn't see it, at all or properly, might want to try another computer or visit a Barnes & Noble to see if they have better luck. It's also always possible the cable's bad.
Is there a reason you aren't using Overdrive? So much easier...
are you talking about the ADE on your PC not recognizing your HD+ or the ADE already on your device not recognizing it. The ADE software on your PC will not recognize your HD+ as Adobe has stopped supporting it. However, there is ADE on your device in the settings that you can enable ADE with so you bypass the PC software. As long as you have an ADE account and have signed in correctly, it should work.
If the on device ADE is not working, then you may have used up your activations. You need to call ADE and ask for more.
Overdrive is okay if you don't mind reading on it. It's not as nice as the stock reader. I prefer to not use it except for public library books as it handles them better. Now that we have the google app store, aldiko is a nice alternative to OD as you can set up your public library as a store in aldiko and the reader is better.
What Compulsive Reader wrote.
HD and HD+ are not recognized by ADE.
You have to go to Nook HD/HD+ Settings and enter your ADE account information.
Then download books to ADE. Then transfer them via USB from PC to Nook HD/HD+.
You can open them fine.
Note: You must register FIRST on Nook HD/HD+. BEFORE you transfer files over.