i figured making pixelmon sidemods would be a relatively popular search so atta give it its own tutorial and since its really not much effort lets go for it.
create a folder in eclipse package explorer named "libs" (you must name it libs otherwise you would'nt be able to build your mod via gradlew) in eclipse package explorer and place your mod of choice's jar file inside like you can see with pixelmon right here.
then right click your mod of choice, goto build path then add to build path so your mod of choice turns to look like so
now in main.java add a dependency und