So I installed the trial version and used it until the time ran out, then I bought Directory Opus and now own it. I am having a problem where Directory Opus is not the default file manager, it has not replaced Explorer. Please help! I have tried re-installing opus but to no avail. 
If you have not done so already, please try this:
From the main menu, select Settings > Preferences > Launching Opus > Explorer Replacement
You can set things here.
Explorer Replacement is only in Opus Pro. If you're using Pro, Chuck's reply above should have all you need. But if you evaluated Pro and then bought Opus Light, the settings won't be there.
During evaluation you can switch between Pro and Light freely to see which version is right for you. A summary of the main differences is here:
gpsoft.com.au/program/pro-lite.html
If you have purchased Light and now want to move up to Pro, you can do so for just the price difference.