Go to your profile, find the "Edit my Profile" button at the top right. Big black button, hard to miss.
When you click on it and scroll down some you'll find the "Edit my About Me page" button. Click there, enter text, and save.
For safety: Remember to copy your text to your clipboard before clicking the save button in case it doesn't work properly so you can paste it in again. Be sure to check your profile to see it has been updated.
Discordian's post in Need someponies help was marked as the answer