Hi Luis,
There are two ways to link a source to step in Convertigo Sequencer. The first way is by using the source picker described here :
www.convertigo.com/docs/CMS/5.2.0/ae1025660.html#Rae50507
Since version 5.3 a new view has been added called "source picker" having a more drag and drop style of operations. Unfortunately this view is not yet documented.
A video of this view operation should be shot shortly. meanwhile , you can use the standard picker wizard for the sequencer projects.
Hope this helps !