Can you tell me the following:
Is your DSV object a named query, a table, or a view?
What column properties show up in the popup when you try to sync descriptions?
What version of SQL Server is your relational database (with descriptions) in?
Can you run Profiler against SQL Server and trace the SQL BIDS Helper generates and let me know which statement is the problem?

I read our comment (named query is not allowed) ... it's a table. But I try this on other computer ... and it's work OK. Sory.
But named query with the same names of columns not work .... may be in next version

changing to a feature request for it to work on named queries

It woulod be good / even more useful if the descriptions on the DSV columns could also be sync'd & leveraged in a similar fashion.
i.e. The ability to sync DSV description properties from the extended properties in the database (as per current 'sync description' on a dimension), and then a 'sync description' action on the dimensions could (should) sync up to the DSV column descriptions (instead of bypassing it and going straight to the source database extended properties).

Doing it this way would provide the added benefit of being able to pick up the DSV descriptions entered on "named calculations" and "named queries" when syncing descriptions on dimensions...