The regular use of the + (plus) sign appears in John Widman d'Eger's Commercial Arithmetic, published in Leipzig in 1489. However, the plus and minus signs did not represent addition or subtraction, or positive or negative numbers, but the excesses and deficits in business problems (Cajori vol. 1, page 128).
The positive and negative symbols came into general use only in England after they were used by Robert Recorde in 1557. However, they were already used before they appeared in writing. For example, they were painted on drums to indicate whether the drums were full or not.
The ancient Greek mathematicians, as noted in Diophantus's work, merely indicated the addition together by the plots - a system that we still adopt today when we want to indicate the sum of an integer with a fraction. As a sign of operation the Italian algebraists used the letter P, the initial of the Latin word. plus.