Ubuntu requires some development libraries for the R packages that can be installed with apt-get in the terminal: sudo apt-get install libcurl4-openssl-dev libssl-dev libxml2-dev
The path to the R folder to be entered in KNIME should be: /usr/lib/R.
Note: In newer versions of FSK-Lab, there is also the option to set the name of a Conda (https://anaconda.org/) environment containing an r-base package. If you have Anaconda or Miniconda installed you may set up an R environment to be used by FSK-lab.