LV2 plugins can be loaded in an effect slot using the slot's down arrow. The LV2 plugins appear in the effect selector's "VST and LV2 Plugins" section (Windows) / "AU, VST and LV2 Plugins" section (Mac).
Windows: LV2 plugins are folders, the name usually ends with .lv2.
64 bit plugins are located in the C:\Program Files\Common Files\LV2\ folder.
32 bit plugins are located in the C:\Program Files\Common Files\LV2\ (if your Windows version is 32 bits), or the C:\Program Files (x86)\Common Files\LV2\ folder (64 bits Windows version).
64 bits MultitrackStudio can only use 64 bits plugins, 32 bits MultitrackStudio can only use 32 bits plugins.
Mac: LV2 plugins are files, the name usually ends with .lv2. They're located in the user or system Library/Audio/Plug-ins/LV2 folder.
You can create the LV2 folder yourself if it's not present.
Tip for Mac users: LV2 plugins that haven't been updated to Apple Silicon yet can be used on an Apple Silicon Mac if you open MultitrackStudio using Rosetta: find MultitrackStudio in Finder, Ctrl-click it, choose Get Info and check the Open in Rosetta option.