1. Uninstall DonorWise. Then download the latest "SetupDonorWise.exe" to your desktop, right-click on the file and click on "Run as Administrator" to install DonorWise.
2. Check that "DonorWise.PluginManager.dll" is in the same folder as where you installed DonorWise. Usually "C:\Program Files\DonorWise\" on 32-bit Windows, or "C:\Program Files (x86)\DonorWise\" on 64-bit Windows.
3. If you still get the error, try manually registering the file. To do this, you need to launch the "Command Prompt" as an administrator. On Windows 7, you can click the "Start" button then type "cmd". Right-click on the shortcut to launch the command prompt and click on "Run as Administrator". Then type in the following:
32-bit Windows:
C:\Windows\Microsoft.NET\Framework\v2.0.50727\RegAsm.exe "C:\Program Files\DonorWise\DonorWise.PluginManager.dll"
64-bit Windows:
C:\Windows\Microsoft.NET\Framework\v2.0.50727\RegAsm.exe "C:\Program Files (x86)\DonorWise\DonorWise.PluginManager.dll"