Effective 1 January 2011, we will no longer support any Certificate Signing Request [CSR] generated with a 1024 bit key. This is because NIST, PKIX, WebTrust and other respective security standards no longer consider the 1024 bit key size as secure. Read more > [1]
A CSR is a file containing your certificate application information, including your Public Key. Generate your CSR and then copy and paste the CSR file into the web form in the enrollment process.
Links:
[1] http://www2.digi-sign.com/about/announcements/2048