Thanks to Rachel for the ssapi.dll file. Here is the code that worked for my:
using SourceSafeTypeLib;
VSSDatabaseClass vssDatabase = new VSSDatabaseClass();
vssDatabase.Open("VSS database path", "userName", "password");
VSSItem item = vssDatabase.get_VSSItem("file path as shown in vss", false);
item.Checkout("Comments", item.LocalSpec, 0);
using (StreamWriter sw = File.AppendText(lblSourceFile.Text))
{
sw.WriteLine("text");
}
item.Checkin("Comments", item.LocalSpec, 0);
vssDatabase.Close();