The instructions in HowToBuildFromSources are for the old squeakvm. For Cog, use the mvm script under the various build directories based on platform/wordsize/ISA and then product/interpreter/object-memory. Here you will find the plugins.int and plugins.ext files. Now choose the build directory for your build variant (production, assert, debug etc.).
Example,
$ cd build.linux64x64/squeak.cog.spur/build $ ./mvm