Explicit credentials require using the Authentication/UseExplicit element in the .udcx
Example:
<udc:Authentication>
<udc:UseExplicit CredentialType="NTLM">
<udc:UserId>domain\user</udc:UserId>
<udc:Password>mYpAssWorD1</udc:Password>
</udc:UseExplicit>
</udc:Authentication>
Using SSO is a better option, as it is more secure since the credentials are not exposed using plain text.
You can find details regarding UDC files at http://msdn.microsoft.com/en-us/library/ms772017.aspx