Jon Medernach has been appointed East Coast sales manager by MRSI, supplier of dispense and assembly systems for the semiconductor and microelectronics industry.   . . .   Tom McCartney has been named president of Avnet Kent, the Americas-based interconnect, passive and electromechanical division of Avnet Electronics Marketing. He replaces Larry Olson, who is leaving for personal reasons. Olsen will join Avnet's board when he departs in March. . . .   Narain Arora, vice president and chief scientist at Simplex Solutions Inc., has been named a Fellow of the Institute of Electrical and Electronics Engineers in recognition of his influential work in integrated circuits modeling.