Make a new folder in OMSI 2/Vehicles/ named Solaris Electric
Download Solaris Megapack and unpack contents of Solaris Urbino PL/ folder into the newly made Solaris Electric folder. Also copy the contents of Fonts/ folder from the same archive to OMSI 2/Fonts/
!IMPORTANT! Delete all .bus files in the folder (In windows, you can insert into the top right search *.bus, while you're inside the Solaris Electric folder and it will list all .bus files there). If you skip this step you will have duplicate of Megapack buses in your game!
Now download Solaris Urbino Electricfamily and unpack the contents of Solaris Electric/ folder to your Solaris Electric folder. it will ask for overwrite, click yes/confirm all.