Once you register her to AHS' website, you need to click the "この製品に関するお問い合わせ", which is an enquiry page.
It should already have picked "VOCALOID Series" for the drop down so then just pick which OS you're using, then put in the name of the product and the final box "★サポート番号" is the support number. When you got Iroha you would've gotten a code to register her and the other one is a support code (on separate pieces of paper). Input that and just ask in English for an import code. They have English-speaking support staff usually so they should reply in English.
The import tool is already provided on that "Update" button in the picture above.
This is all done and located on your MyPage from when you registered her on AHS' website.