I have just downloaded Blackberry Messenger V7 and i keep getting a message that says
You are attempting to open a secure connection, but the server's certificate is not trusted
Then there are options to Continue, Close Connection, View Certificate, Trust Certificate

What should i do to get this message sorted out?

That message is not coming from BBM. View the certificate and check the source. If it is valid, trust the certificate.

I have tried to accept the certificate and the message keeps on coming back
Is there anything else i can try?

This is a known issue with BBM7. Try searching the BlackBerry support forums.