Don't have a nook color, but I do have general experience with this stuff.
Tell us what you've done so far.
First, download CM7. Either nightly (Which are the most recently updated versions, but do not promise to be stable [for testing various features and stuff]) or the RC build. I would go with the RC build for now.
Then download the image file from the link in the guide (The first one). You will then install this image file via a third party program. May I recommend Win32 disk imager?
Now, write the image file to the card via Win32 disk imager. Make sure you've formatted your card beforehand (Also meaning backing up your files). It should be a pretty straightforward process.
Now take that CM7 rom you've downloaded and put it in the root of your SD card (Not in any folders in the SD card). Make sure it's a .zip and you haven't renamed it.
You got this part yet?