This may simply not be available/slated for free outside the UK. Even though Mills & Boon is the UK spinoff of Harlequin and they mostly share a common catalogue and MIRA is primarily an HQN imprint, they do tend not to share freebies across the pond, and this doesn't yet exist in the worldwide Harlequin catalogue, even though they've got some of O'Brien's other historicals.
It's free @ the
Mills & Boon site, for anyone who wants to try downloading from them if they accept non-UK customers.