If you can not find the DRM key saved to your computer, most likely it is hidden due to security reasons. Check your computer settings to be sure you're showing the "hidden files and folders". If you are not showing the hidden files and folders, do the following...

Step 1 - Open up "My Documents".
Step 2 - At the top of the page, click on Tools".
Step 3 - Now go to the bottom of the tools menu and click on "Folder Options...".
Step 4 - Click the "View" tab.
Step 5 - Underneath the heading "Advanced Settings", find "Hidden files and folders" and select "Show hidden files and folders".
Step 6 - At the bottom you will need to click the "Apply" button and then the "OK" button.
Step 7 - Now go back and find your DRM key. If you still cannot see it, then you have not actually saved the DRM Key, or you saved it somewhere else.