I don't know how Cubase 5 differs from Cubase 9 as far as menu options, etc., how familiar you are with Cubase, and whether you've already tried the following steps. They're for Cubase 9, but hopefully Cubase 5 is similar.
Click on the "Devices" menu and select "Plug-in Manager."
When the "Plug-in Manager" window pops up, click on the little gear button in the bottom left corner (if it isn't already selected) to toggle to the "VST 2 Plug-in Path Settings" frame.
Rather than moving or copying Kontakt's folder to Cubase's folder, it would be better to tell Cubase where to look.
If Kontakt's folder isn't already listed among the paths, click on the "+" icon ("Add Path") in the upper right corner of the "VST 2 Plug-in Path Settings" frame.
When the new popup window appears, browse to Kontakt's folder, select it, and click on the "OK" button.
Back in the "Plug-in Manager" window, click on the "Blacklist" tab and see if Kontakt is listed.
If it is, click on it.
Click on the little "i" button in the bottom left corner of the "Plug-in Manager" window to toggle to the information frame.
If the message says "This plug-in has been blacklisted," click on the "Reactivate" button.
When the warning pops up, click on the "Reactivate" button.
If a message says "Reactivating the plug-in failed!", you'll need to contact the vendors to see if a solution exists.
Otherwise, if reactivation did not fail, click on the little gear button again to toggle back to the "VST 2 Plug-in Path Settings" frame.
Click on the little circular icon (two looped arrows, "Rescan All") in the upper right corner of the "VST 2 Plug-in Path Settings" frame to rescan the plug-in paths.
Hopefully the plug-in will show up successfully now. If it doesn't, you'll need to see if you can determine the problem and correct it. Note that once a plug-in has been blacklisted, Cubase will no longer try to load it on startup, so it's possible that the first time you started up Cubase after installing Kontakt, something prevented Cubase from succesfully loading the plug-in and it was blacklisted, such that from then on it looked like Cubase simply wasn't finding it. Hopefully you'll be able to resolve the issue so Cubase can load Kontakt successfully.
For instance, I just tried to start Cubase 9 but it got stuck trying to load a plug-in from FL Studio-- there was a popup message from FL Studio about a new update being available, so the loading process timed out. I had to use the Windows Task Manager to end the process for FL Studio, as it was frozen, after which Cubase was able to continue starting up. Consequently, the FL Studio plug-in was blacklisted. I exited Cubase, started up FL Studio (which is just a trial version, anyway), and told FL Studio to ignore the update and not to display the message again. Then I exited FL Studio, started Cubase again, reactivated the blacklisted FL Studio plug-in, rescanned, and it was successful.