The SpeedTree Modelers for UE4 and Unity require a subscription to unlock the full editing capability. These subscriptions can be purchased at the SpeedTree Store.
However, even without a subscription, the Modeler can be used as a tool to prepare trees for these engines. These trees may be ones that you have bought from the SpeedTree Store, or ones you have made yourself before your subscription was cancelled. An account is still required, which you can aquire at the SpeedTree Store, for free. Once you enter this information into the login screen of the Modeler and no active subscription is found, you will be prompted if you wish to use the Modeler in Export-Only Mode.
In Export-Only Mode, the Modeler disables all editing features. You will not be able to add, remove, or change any of the geometry in the tree. Materials, meshes, and other assets will be locked. No major wind effect changes will be possible.
But, you can: