You cannot directly modify members of Padding and Margin. Properties are not the same as fields.
You can only get and set the value of a property. You can modify the margin properties using the following convention:radComboBoxItem1.Margin = new Padding(0, 1, 0, 0);
This issue also described here:http://msdn2.microsoft.com/en-us/library/wydkhw2c(vs.71).aspx
Hope that helps. If you have any additional questions, please do not hesitate to contact us.
All the best,
the Telerik team