In order to gain access to editing approvable items within your project, you will need to be added as a user on that project.
Users can be added/removed within the project settings tab of the individual project. Users can only be added by company Admins or a user with Project Manager permissions on the project.
For subcontractors, we recommend using PM, Editor, & Contact for your project permissions. With these permissions, you will have full access to the project, receive all email notifications, and will be able to edit and submit all approvable items.