Lmms is not ready for that update. You may be able to run it as vst, like i do with the previous version, but i do not know if they have a vst-version of this release https://www.youtube.com/watch?v=hrrl3OiiJvM
That's the FLTK UI. Actually, if you can compile LMMS, you should be able to just use "cmake -DGuiModule=ftlk .." when configuring the project.
Additionally, there's a brand new UI now. It can, however, currently not be used with LMMS. Maybe with LV2 support, or maybe even earlier, who knows...