You do not need to register the assembly at all to use it from PowerShell. Registering is needed for COM only (e.g. VBScript, JScript, VBA, etc).
I'm getting the exactly same error message, when the assembly is blocked only. Are you sure that there's no Unblock
button on .dll's Properties window in Explorer? Are you sure you are loading a correct copy of the assembly? Did you try using a full path?
You can also try to debug loading of the assembly, but it's rather advanced task: