If you have a DTN.IQ subscription, you can use 3rd party software that is compatible with IQFeed (you will still need to install the IQFeed client however).
There is no API for SmartMarket or Custom Scans.
Maybe you are looking for some other kind of documentation, but the DTN.IQ Help Manual is available on the website under support:
http://www.dtniq.com/product/help/v2.4/
Jay Froscheiser
DTN Market Access, LLC.