The XML string for both of these can be found in your mojoportal installation under App_GlobalResources/Resource.resx

You can always simply change the text there, but you'll have to keep doing it every time you update mojoPortal because your changes will be overwritten.

However, Joe Audette wrote an article about a method for changing these without forking the code not long ago. I would suggest checking it out. Here's the link: http://www.mojoportal.com/a-clever-trick-for-customizing-labels-and-buttons-in-mojoportal