Oh install bepinex don't install the api since the mod menu is not for 0.10 or 0.11
and it only works for 0.6-0.9
and bring everything in the BepInEx folder and drag it to your BB+ folder
launch the game and close it with alt+f4
then the plugins folder appear in the BepInEx
in the oldsport mod menu folder drag the UniversalModMenu and the Newtonsoft.json dll then drag it to plugins if the plugins folder does not appear create a folder named "plugins" inside the BepInEx folder