Install it on your computer (which you have successfully done)
which is like buying a book and putting it on your bookshelf
And also access it in your session with
library(package_name)
in your case, this would be
library(difR)
this is like taking the book off the shelf and opening it
Now you can use the functions in the package.
You may be missing the library step.
Alternatively, you can work around the library step (if you have the package already installed)
by specifying the package each time you call a function, with the double colon structure.
One example with the readr package:
readr::read_csv("my_file")
this will read in a csv file
If you are using functions from a package more than once, it is easier to do the library step.