Vinu,
We have a set of special debugging request headers you can use in the
sandbox for testing this functionality (see
http://code.google.com/apis/adsense/developer/adsense_api_sandbox.html#requestheader
for details).
You'll want to specify these extra three SOAP headers when issuing
your createAccount request (setting debug_association_type to
NotAssociated and the others to whatever values you want). That sets
up an account on our end with the specified data. Afterwards, you
should be able to call associateAccount with the postal code or phone
number that you set.
Cheers,
Wes