Add-Type -Path c:\scripts\validateMatches.dll failing.

I can get Add-Type -Path c:\scripts\validateMatches.vb to work with the script below but trying to reference the compiled version  fails. This script just adds a [ValidateMatches] type and is identical to [ValidatePattern] but with the extra message parameter. This was converted to VB.NET from the C# source first published at www.huddledmasses.org/jaykul/.The Test-Backup script is as follows:

function Test-Folder {
   param([ValidateMatches(Options='None',
                          Pattern='^[a-z]:\\.*[^*\\]$',
                          Message="Please enter a valid path like: 'c:\Modules\library'")]
         [string[]]$folder = $pwd
   )
   write-host $folder -fore cyan
}
Test-Folder 'd:\scripts\modules','C:\users\fred' #Use Upper case C here to trigger error.

The Option='None' is identical to that in [ValidatePattern], poorly documented at Microsoft, and just forces Case Respect. If I try and reference the compiled dll the following error is  generated:

.\Test-Backup
Cannot find the type for custom attribute 'ValidateMatches'. Make sure that the assembly that contains this type is loaded.
At c:\Scripts\test-backup.ps1:7 char:18
+        [String[]][ValidateMatches(Pattern='^[a-z]:\\.*[^*\\]$',
+                  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : InvalidOperation: ([ValidateMatche...les\library'")]:AttributeAst) [], RuntimeException
    + FullyQualifiedErrorId : CustomAttributeTypeNotFound

So what simple setting is missing please?