Hi Dez!
I normally use R on Windows, but I have a Macbook sitting around and I tried re-installing dplyr and got exactly the same errors/behavior as you, so it's not just you. To expand on what andresrcs recommended, I was able to resolve the error by specifying a particular CRAN mirror as an additional argument to install.packages(). Here's what I ran:
install.packages("dplyr", repos = "http://archive.linux.duke.edu/cran/")
If that particular mirror doesn't work for you for some reason, you can get a full list here so you can try others:
https://cran.r-project.org/mirrors.html
Remember to load dplyr after installing using library(dplyr). Also take a look at the very helpful dplyr cheatsheet here:
Good luck with your class!