capturing cassettes from new device

I have a new usb capture device and it gives me Warning that says Error opening sound device. Please check device settinngs and rate…It came with a mini disc and I can’t use in my IMac as it has a horizontal slot drive. Apparently I need the plkug-ins. Can you help. I have El Capitan and AudCITY 2.1.2

What is the make and model number of the device? If it is designed for video capture, Audacity probably won’t be able to use the audio in its USB stream. You would have to take the headphones out or line out of the device to the stereo line in of the mac. If it has a stereo line in there will be a circle and two arrows as illustrated below:

What plugins are you referring to?