From what you describe, it sounds like you have created an 'Application Extension' app in App Center, then added your Custom Activity to that app.
While this is suitable for development purposes, it does not enable the app to be used by any User — it can only be used by the User Account that you selected when you created the app in App Center.
Assuming that you want to make this available to other Users in your defined account, you need to contact your Salesforce Account Manager and order an additional feature license to enable your Custom Activity to appear for all user accounts. The feature is named Marketing Cloud Custom Apps and the SKU is #200400.
For provisioning purposes, you will need to provide Salesforce with the app 'Application ID' which you can find in the Summary section on the Overview page of your App in App Center.
Once enabled, you can then define which Users can access the Custom Activity in Journey Builder, by opening 'Marketing Cloud Extension Manager' from the Platform menu as indicated below.