When a scheme is sent to you for signing, a green sign-off button should be visible at the top right of your screen.
If you can only view the plan but cannot sign the plan off, then this might be due to your user permissions. When your trading partner invites you to Collaborator, they have the ability to choose whether you are able to only view the plans, or you also have the ability to sign off plans. If you expect to be able to sign off a plan within Collaborator, please contact your trading partner for them to change this setting.
When signing off a plan, the plan reference must be typed into the text box before you can proceed. Take care when typing this plan reference - it must match exactly including capitalisation of letters.