I agree with the other posters.
There is a French version of the Arbeitgeberbescheinigung form called the "Attestation de l'employeur" which may be easier for your previous employer. http://www.espace-emploi.ch/dateien/...usfuellbar.pdf
Get your previous employer to fill it in and stamp it and they should also send you back details of your earnings. If they don't do this you'll have to send your last 6 months payslips with the attestation de l'employeur to the Swiss unemployment office. That should be all you need but I've heard stories of them asking for a copy of your AVS card and a copy of your passport as well.
You'll have to send the form to the "caisse de chomage". I think any caisse de chomage will do it, not just the one where you worked.
You can find the addresses of the caisses de chomage here: http://www.espace-emploi.ch/downloads/adressen/
(which is the same site that Ittigen linked to)
I was under the impression that your previous employer is obliged to give you the attestation de l'employeur when you finish working for them so they ought to know about this form.