EchoSign supports static XFA forms, but dynamic XFA forms will not be processed at all unless flattened first. Regular PDF forms can be processed by Echosign and most Acroform fields will be converted to EchoSign form fields, especially if using Acrobat 11, convert to EchoSign feature.
Also Echosign is not a PDF Reader, when you get to the signing stage you see a high quality raster of the uploaded file with EchoSign fields overlaid on top.
For static XFA you cannot use the new text tag format as indee this syntax included ':'. You can use the old style text tag instead for naming your fields, though the old style does not allow for recent feature additions. I've attached an old tagging guide.
A better way of using text tags in LC designer is to use text areas and use current text tags with the {{ }} and colons instead.
Or if you lucky enough to have access to a complete LC server suite, you can build a workflow which ends by flattening a dynamic XFA/XDP and send the completed but flattened PDF to the Echosign API to get a final signature.