The next nopCommerce version will support Medium trust. But if you don't want to wait, look at http://forums.nopcommerce.com/forums/default.aspx?g=posts&t=224 this topic
You need to recompile Nop.DataAccess.SqlServer (after removing the reference to Microsoft.Practices.EnterpriseLibrary.ExceptionHandling.Logging.Configuration.Design)
Then just upload the updated dll into your site's bin directory.