You don't tell us what you tried... Have you read the http://wiki.processing.org/w/How_to_Install_a_Contributed_Library article?
I tried to download the ReactP5 library. I see the folder in the zip file is named yconst-ReactP5-b365a01. Have you renamed it to ReactP5, ie. the name of the jar file?

And, please, find better subjects than this one, it is totally unhelpful. And the all-caps subject is looking as if you are yelling.

I did indeed change the names of everything. Was still getting no joy. Have being sick last few days so head a bit slow. Will take another look tomorrow. You get it running for you?