Problem using HTTPS

Hi,
I'm trying to set a local station in http.
The first thing I should do is importing in SSL/TSL the public certificate, and right here I have the first proble, because each time I import it, using import certificate or "import your own provate key (from PKCS#12) I see the message "This key is valid", but the trust anchor is considered "untrusted". I try several ways, but each time I have the same results.
When I go to the "partner configuration" section and tab security I cannot find the certificate and the only way is to add the certificate in "Sign/cript" certificates. The problem is that when I receive the test message from my partner (who has my public certificate) I receive the error: Outbound MDN details: Error decrypting the inbound AS2 message: Recipient certificate does not match.
Can you help me to follow the right direction?
Thanks