When you receive your certificates from Digi-Sign there will be your site certificate (named yourdomain.cer) plus 2 others (UTN-USERFirst-Hardware.crt and
Digi-SignCADigi-SSLXp.crt or Digi-SignCADigi-SSLXs.crt), these 2 must be installed as a Trusted Certificate Authority [1] CA and Certificate Chain.
Links:
[1] http://www2.digi-sign.com/certificate+authority