Find your version of SPSS (18 through 24).

Download and install the SPSS-R Plugin. You can find the relevant installer for your version of SPSS at this Github page. This blog post from IBM also has more information on installing the R plugin.

For SPSS versions 18-21, download and install the SPSS-Python Plugin. For SPSS 22+, this plugin is installed by default.

Download the CEM for SPSS extension bundle: CEM Extension Bundle

Open SPSS, go to "Utilities > Extension Bundles > Install Extension Bundle..." and direct the dialog box to the "cem.spe" file you downloaded.