can you please make sure that the Category metafield is registered for the Images&Documents module. To do so, please do following:
1. Open your web.config file
2. Make sure that this line is present as the child of telerik/cmsEngine/metaFields
add key="Libraries.Category" valueType="ShortText" visible="True" searchable="True" sortable="True" defaultValue="" />
If the line is not present, paste it there and let us know how it goes.
All the best,
the Telerik team
Check out Telerik Trainer
, the state of the art learning tool for Telerik products.