You can find an example on how to sign a text using the private key in C# example in the SDK.
Once you have imported a pfx you can sign by this snippet :
// prepares for the signature
string helloworld = "Hello World";
byte[] text = Encoding.ASCII.GetBytes(helloworld);
// launches the digital signature operation with a RSA_PKCS mechanism
nRes = session.SignInit(Mechanism.SHA1_RSA_PKCS, privateKey);
// computes the signature
byte[] signature = session.Sign(text);
// launches the digital signature verification with a RSA_PKCS mechanism
nRes = session.VerifyInit(Mechanism.SHA1_RSA_PKCS, publicKey);
// verifies the signature
nRes = session.Verify(text, signature);
// results if nRes == 0 means that the verification is OK
Console.Write("Verified " + (nRes == 0));