Sorry but, by the 'latest version', do you mean the current stable release (6.0.7), and not a development build? I shall assume that you do.
To erase free space, under Windows 7, you will normally need to run Eraser as administrator. Instructions for doing this are in the 'Getting to know Eraser' FAQ post.
Before you do this, start the Eraser program, and make sure that you are looking at the Schedule. It may well be that, in the Schedule, there are one or more entries relating to the task you tried to run; it/they could be marked as 'completed with errors'. If so, right-click on one of these entries, select 'View Task Log', and look at the error message. If it refers to the need to run as administrator, or if there is no log entry or error message, run the program as administrator as previously indicated, and try the erase again. If there is a different error message, please could you say what it is.
David