Future versions of Viscosity will provide a nicer way to install icon packs. For now you can install an icon pack like so:

1. Download the icon pack (e.g. Lock.zip)
2. Double click on the zip file to uncompress it. You should see a folder appear in the same location (e.g. Lock.ViscosityMenuIcons)
3. Right-click or control-click on this folder and select Copy (e.g. "Copy Lock.ViscosityMenuIcons")
4. Find the Viscosity application (usually in your Applications folder)
5. Right-click or control-click on Viscosity and select "Show Package Contents"
6. Open the Contents folder, and then the Resources folder
7. Find the Menu Icons folder and open it
8. From the Edit menu select "Paste Item"
9. Reopen Viscosity and set the menu icon under Preferences->Appearance