Before running MTK, you must first specify the directory were the plugins - the plugin_name.pgn files - are located. This can be done by setting a MTK_PATH environmental variable to point to this location. Since these plugins will usually be located in the $TANGRAM2_HOME/lib/mtk_plugins/ directory, all you have to do is insert a line, similar to the one bellow, in your .bashrc file.
export MTK_PATH=$TANGRAM2_HOME/lib/mtk_plugins/
If, for some reason, you have other plugins located elsewhere, or just want to specify a new directory without changing the MTK_PATH variable, you can use the -p directory option, in the command line, when running mtk.