Anyways, ran in to this one while configuring an Office Web Apps 2013 farm the right way using a certificate:
I had a certificate added to Certificates (Local Computer)\Web Hosting figuring that is where IIS would be looking for it when I created a new OWA farm.
But when I ran this:
New-OfficeWebAppsFarm ... -CertificateName "TheCorrectCertificateFriendlyName"
I got this:
New-OfficeWebAppsFarm : Office Web Apps was unable to find the specified certificate.
Solution:
Add the certificate to Certificates (Local Computer)\Personal because the command will find it if you do.