Sending a letter of demand. I do not have current address, do I send registered to place of work, do I send email , which may be ingnored.

Unless NH has some unusual procedure of which I am not aware, a "letter of demand" is not a legal procedure, therefore  you can send it any way you want.  However, I'm not sure why you're bothering with a demand letter if your adversary has already told you he has no interest in negotiating a settlement.  If you commence a lawsuit, you will have to serve the defendant by one of the means allowed under NH procedural rules, which probably allow you to serve the defendant at his place of employment.  If you don't know the rules, and are not using an attorney, you can usually find the rules online, or the clerk of the Court can help you.  The same applies to an arbitration.  If you are referring to a demand for arbitration, you must serve in accordance with the rules of the arbitral body.  If you can't find the applicable rule (most prominent arbitration forums, such as JAMS or the AAA, post their rules online), you can call the organization to get  help.