Without knowledge of the specific API it is impossible to tell you how to call it – other than to say “via a prototype”. If you provide details of the API we can probably give you a hint or two.
As to the “best” XML parser – well it depends. Best as in low cost? Best as in speed? Best as in …..
Check out Scott Klement’s recent series of articles on using expat from RPG (www.iSeriesNetwork.com and search for Expat). It is not a very sophisticated parser, but it is free and very fast.
Check out Aaron Bartell’s www.mowyourlawn.com for another option. He has wrappers for both a DOM and a SAX parser there.
There’s also IBM’s XML option including DB2 XML extender. These are all “for fee” options, but not very expensive – and you may already have them.