Greetings!
The Default\admin is usually the issue, unless they have changed the default SEPM domain from Default, in which case you need to use THEIR SEPM domain name there. As you know this isn't thier Active Directory Domain Name, unless they have changed it from Default to their Domain Name to mirror their environment.
The other thing you need to look at is the AUTHORIZATION to connect to SPC. Now that I write this, it seems this may actually be your issue.
Make sure that you have enabled the integration of SPC inside of the SEPM Manager.
To do this:
1) Navigate to Admin/Servers/Local Site and click Edit Site Properties.
2) On the Web Services Tab make sure that you have Enable Network Discovery enabled.
Another thing to look at is the Admin/Servers/Local Site/Your Server Name properties.
1) Navigate to Admin/Servers/Servers/Local Site/Your Server Name, Properties
2) Make sure that on the General Tab you have the SPC Server added to the Server Communication Permissions, or make sure that all consoles and servers are granted access to the SEPM server.
Below are the relevant Integration Guides and a good resource for finding documentation.
SEPM Integration Guide
http://www.symantec.com/docs/DOC4498
Finding Documentatioin
http://www.symantec.com/docs/DOC4852
Here is the article about verifying the Domain Name that is to be used on the integration line:
http://www.symantec.com/docs/TECH166235
The other option is to call support.
Let them help you out. I know they would be happy to.