But I'm supposed to be able to backup my Windows account security credentials (a decryption key or something of the sort) and import it on another machine to use it, right?
Is there some step-by-step guide to do this?
The operawiki.info page is for transferring data to another Presto version, and not useful here. The desktop blog link is the one you want. If it doesn't work, then you'll have to delete your Opera 20 profile as well to force it to re-import the data. As in, close Opera, delete the Opera Stable folder from your roaming directory, open a Run dialog or Command Prompt and type
launcher.exe –presto-master-password=<master password>
... of course with your actual master password where it says <master password>