Insert my own plugin dosent work in multisite

I was trying to configure my own SIMPLE plugin following the instrutions in codex and also in this link
What i really want is to have my on plugin with its css stylesheet in order to customize my admin panel.
But it doesnt work. I mean it works, i have changed diferent styles and it works but the problem is the site behavior. Comes out some messages above the site both in the front end and in the back end. And i followed the steps verbatim.
Maybe is because i am in multisite, and i have to change anything?
Thanks for your help