Web SSO SAML Assertion login to saleforce.com

Question

We have MVC 4 application from that we are receiving Auth token from saleforce.com to use their API using following code

requestData.Append("grant_type=password&"); requestData.Append("client_id=" + ConfigurationManager.AppSettings["SfdcClientId"] + "&"); requestData.Append("client_secret=" + ConfigurationManager.AppSettings["SfdcClientSecret"] + "&"); requestData.Append("username=" + login.Email + "&"); requestData.Append("password=" + login.Password);

Everything is working fine so far
Now we are implementing SSO using Siteminder which will do all authentication process and return user id on successful Authentication
So now we have only User ID with us and want Access token from Saleforce.com using saml assertion
We form Saml request and posting it to saleforce.com as per their documentationhttps://developer.salesforce.com/page/Digging_Deeper_into_OAuth_2.0_on_Force.com but its giving invalid_Assertion error we try to figure out but still not get any solution to this
Does any one know have to do this please assist me