Did I extract the file incorrectly or something?
Fred is steering you to the corect procedure in his reply above, Erik. The Crystal plugin is a VSTi; unlike the DXi plugins that you see listed in the default P5 installation. You have to 'convince' the plugin to act as a DXi, and that happens in the VST Adapter.
First of all, you don't really want to start loading your VSTs into the P5 folder. It won't do any immediate harm that I know of, but you're going to start bogging down the P5 folder eventually with all sorts of files that accompany the VST .dlls, and it'll become impossible to retrace your steps later on. This has more to do with good housekeeping.
My suggestion is to create a top-level folder for just your VST plugins. It could be as simple as C:/VST Plugins, and you could divide it down even further with C:/VST Plugins/VSTi Synths and C:/VST Plugins/VST Effects. Under the C:/VST Plugins/VSTi Synths sub-folder, create another folder named Green Oak and extract the contents of the .zip file to that folder. Since you already done that to the P5 folder, you might want to delete those, or move them to C:/VST Plugins/VSTi Synths/Green Oak in Windows Explorer.
That'll include the CrystalPatchBanks and CrytalSoundFonts folders (if you downloaded the entire package), along with the Crystal%20VSTi.pdf (manual) and the Crystal.dll (the 'business end' of the plugin). Go to the Start/All Programs menu in Windows, navigate to the Cakewalk folder, and look to running the Cakewalk VST Adapter 4 program. This is what 'preps' your VST(x) plugins for use in Project5 (or Sonar, etc.).
In the second screen, browse to your newly-created VST folder (in my example: C:/VST Plugins). You only need to point the Adapter to the top-level folder; it'll scan for any .dlls in the layers underneath that. Watch the process unfold, and when you're finished, your Crystal plugin will be 'registered' for use in P5. You have to repeat this process everytime that you add or delete a new plugin [VST-based] to tell Project5 of the changes you've made.
There's all sorts of options that you can adjust during the VST Adapter scan. Leave them alone for the time being, but remember that the options are there should any plugin problems develop later on. You don't need to go through this for a DXi synth or DX effect; only for VSTis and VSTs. The next time that you open P5, Replace or Insert Instrument will contain a VST section under the DXi 'line', and under that: Crystal; ready to play.
Enjoy that synth. It's one of the best 'freebies' out there, and there's
tons of patches to download for it around the 'net. Make sure that you grab any patch banks by Tim Conrady (patches usually are appended by "TC" if they're his). He's got some fantastic creations out there, and his method is simply tweaking one patch to get another. He saves that, and starts anew from the freshly created patch. It's incredible how many unique sounds that he comes up with using that procedure, although sometimes you can see the progressive 'lineage'.
If you have any additional problems with this, don't hesitate to reply back. HTH