When you request a certificate, your Mac creates a public/private pair that will be used to download the certificate from Apple. This is why you can't download the resulting certificate from another Mac that doesn't have the original pair.
If you deleted the original pair, you have to re-import it first. If you don't have both halves of the pair, you may have request a new certificate from scratch.
If you have the originals and it won't import them to the login keychain, you might try creating a new keychain to use in the account with the "KeyChain Access" utility. If it imports the information, the Mac should be able to find it as long as you leave the new keychain open.
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…