After you get the property you want you have to set the swCustPropMgr to your active configuration like this:
Dim swConfig As SldWorks.Configuration
Set swConfig = swModel.GetActiveConfiguration
Set swCustPropMgr = swConfig.CustomPropertyManager
swCustPropMgr.Set2 "Description", strValue
The code you have works with document level custom properties. The code above works with configuration specific custom properties. You might also want to check to make sure the custom property exists first. If not, then you will have to use the swCustPropMgr.Add3 method. Hope this helps.