This is just a quick tutorial because I haven't found this information anywhere else on the internet but I'm sure a lot of people are having trouble getting Mouse Assistant 5 to work on their Mac.
Please make sure to use the original software. Do not download it from anywhere else but Elecom's own website: https://www.elecom.co.jp!
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https://www.youtube.com/watch?v=u7FV4z76V_M