To whom do I address the letter?

If an addressee is not stipulated, call the company and ask for a name. If it’s a woman, don’t forget to ask whether she likes 'Dear Mrs, Miss, Madam, or Ms'. If you can’t find out, the fallback is nearly always 'Dear Sir'. Do not panic over the salutation; there are only so many you can use and you will not be penalised unless you have made a glaring error and addressed it to the wrong person. Try and avoid 'Dear recruiter/HR department/manager' and NEVER say 'Hi'.