I've tried the city phone book, though I think my lack of Polish fluency hindered my search since not a single Zajaczkowski turned up!
The Polish spelling of your last name is: Zajączkowski. When searching Polish records online it is very important that you type an Ą
and not an A
Also, Wrobleski is a very uncommon last name in Poland. The website moikrewni lists only 5 people in Poland with that last name (none in the Warsaw area). On the other hand Wroblewski (Polish spelling: Wróblewski) is very common. Are you sure that Marek's last name is spelled without the second w
? There are quite few people named Marek Wroblewski in the US phone directory. Perhaps you can use it to search for your cousin?
FYI: your father's name in Polish: Zdzisław Zajączkowski
According to the website moikrewni, there are 273 people in Warsaw whose last name is Zajączkowski. You need more info, like your father's birth certificate, old letters from the relatives, etc. to locate your cousins.