As far as I know there is only one way to learn it: just do it. Still if you are experiencing pain then it could be wise to have somebody just look at your seiza, just to be sure you arent doing anything funny.
As for westeners not being able to sit in seiza, well I think this belongs in the same category as Japanese seamen being so special one cannt use a western condom or Japanese snow being so special one cannot use a canadian snowplow on it, the category of BS.