Carl Zeiss MicroImaging Inc. and EXFO Electro-Optical Engineering Inc. have signed a value-added reseller agreement that enables Zeiss MicroImaging to sell EXFO's X-Cite 120 fluorescence illumination system with its microscopes in the US and Canada.   . . .   OmniVision Technologies Inc., an independent supplier of CameraChip solutions for high-volume imaging applications, announced that its vice president of finance and CFO, H. Gene McCown, is retiring. He will stay on until a replacement is named.. . .   Jim Hamlin has been appointed sales manager of the Microscopy Division at Nikon Instruments Inc. Hamlin, who was previously Nikon Instruments' western field sales manager, will relocate from Dallas, Texas, to Nikon's Melville, N.Y. headquarters.