I want to get back into Arduino and have 2 older Uno 3 boards. I tried downloading code but nothing is showing up on my iMac. Lots of questions, need step by step guidance. This is a situation where it would be best to speak with someone at SparkFun to guide me through.
Nobody (at Sparkfun or otherwise) will be able to help until you provide a few more details about what exactly you are having trouble with.
You said you are trying to download code on a Mac – but what code is it? Are you trying to install the Arduino IDE itself? or are you trying to install a board package for the Uno R3? or are you trying to install a library? or are you trying to install something else?
Swap cables/ports, or try on a different machine if possible and/or start over here