I don't think WebNIC's v2 API is supported in WHMCS v5.
http://www.webnic.cc/newsletter/032011/ann_201103140661eng.html
New API: https://www.webnic.cc/NewAPIDocument_eng.pdf
Upgrade API: https://www.webnic.cc/UpdatedAPIDocument_eng.pdf
I'm using WebNIC for its competitive pricing for the growing demand in Asia. I use WHMCS to only manage invoices, projects, and hosting, but not domains.