Seems either you're specifying an invalid path, or the SQL Service Account doesn't have permissions to access the directory it's in.
Try using a T-SQL Restore Database command instead of the GUI.
RESTORE DATABASE myDatabase
FROM DISK = 'F:\myDatabase.bak'
WITH
MOVE '' to '',
MOVE '' to '',
STATS = 10;
GO
filling in the appropriate values in the MOVE clauses
and make sure that SQL Service account has permissions to access the file(s)