The Czech letters with diacriticals are these
á, č, ď, é, ě, í, ň, ó, ř, š, ť, ú, ů, ý, ž
Some of them you can get by simply holding down the key on a Mac. for example, holding the c gives you the č (and others to choose from, ç or ć) similarly for š and ž, but does not work for the n or r. Holding the u gets ú as an option (likewise for a, e, i, o).