Step 1: Sign up for an account at https://developers.onemap.sg/signup/
Step 2: An email will be dispatched to your personal email account for verification.
(Please check your junk mail too!)
Step 3: Confirm your OneMap account using the “Confirmation Code” listed in the email.
The confirmation site is @ https://developers.onemap.sg/signup/confirmAccount.html
Step 4: When you are all done, you can retrieve your access token through the authentication web-service listed at https://docs.onemap.sg
Hope this resolved your registration woes!
PS. There is a topic on Access Token retrieval at Steps for API Authentication
Have a great day!