We've made it available for you to add only Contact custom fields in 
Global Data (Event custom fields can be added in Event Builder → Invitee Questions).
Contact Custom fields are available with Business and Enterprise subscription plans (learn more about iClosed pricing here).
Here's a quick guide on how to add Contact Custom Field in Global Data:
Navigate to
Global Data → Contacts sectionScroll to the far right of your screen
Click on "Add Column"
New window for creating a Custom field will show
Select Custom field type and enter Custom field name
Important Notes
Even though we've made Contact Custom fields creation available from
Global Data → Contacts section, we strongly recommend you to create
all custom fields from the 
Tracking menu.
In Tracking menu, you will be able to to see all already created custom fields, which will help you to avoid creating duplicates, which will help you to keep your data clean and consistent across the account, and more importantly, to keep your stats accurate.