There are two ways to add new Topics to your account. Both require that you first have a Subject for it to live in.
Option 1: Open the Subject where you want the new Topic to live. Then, make sure you're in edit mode which will give you options to add Topics and tests at the bottom below existing Topics. Click +Add Topic, name the Topic and click Save Topic.
Option 2: From Subject Outline View, expand the Subject where you'd like to add a new Topic. A text field will appear where you can type the name of the Topic. Once you type the name, click +Add Topic to save.