On the Home screen of the FHA online application select New user for existing application? Register Here!.
Then enter the applicant company ID that was provided in the email when your company was first credentialed.
You will need to talk to the person within your organization that set-up your company to gain access to the email.
The FHA online application is available on the How To Become An FHA Approved Lender link in the Approvals & Renewals section at: https://www.hud.gov/program_offices/housing/sfh/lender/
HUD Policy Determination