DotNetNuke Skins and Resources by ThinkofDesign

Register and Login-Logout text are editable from the DNN admin interface with the language editor.

If you browse the Admin > Languages page and then mouse over the small actions arrow you ‘ll find the language editor.

There you can find the keys you are looking for at:

Local Resources > Admin > Skins > App_LocalResources >

Login-Logout keys are in the Login.ascx page and Register key is in the User.ascx page.