Using xml-rpc to import product data containing custom fields into Marketpress products

Installed Marketpress and then CustomPress. Created a custom field for Vendor for post type product. Id of the field is ct_Vendor_text_866b

This field is displayed in the add product screen in MarketPress and I can use a CSV plugin to import the data and the vendor data is displayed in the Marketpress product admin screen. So far so good.

I then used the wordpress xmlrpc api http://codex.wordpress.org/XML-RPC_WordPress_API/Posts#wp.newPost
to create a new product. For the custom fields section, I added mp_price and ct_Vendor_text_866b to the array.

The api call returns successfully, a new product is added, the title and content fields are set correctly. The price is also populated correctly. Only the field (Vendor) created with CustomPress is not populated.

Any suggestions.

Thanks,
Ash