On the Home screen of the FHA Online Application select the option "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.
For the FHA Online Lender Approval Application please refer to: https://www.hud.gov/LENDERS under the How to Become an FHA-Approved Lender link under the Approvals & Renewals section.
HUD Policy Determination