Can sign up for it here I believe. www.sage.com/.../development-partners-erp-crm-hrms

You don't have to sign up to anything. You can find the complimentary Access to the Sage 50 SDK on this page: www.sage.com/.../development-partners-erp-crm-hrms

Scroll down the page and open up the collapsed sections by clicking the "+" icons. The first section "Program Levels" contains links to the SDK.

Hope this helps.