There are many ways to use いいじゃん, and in a lot of cases the phrase is hard to render accurately in English. So it's well nigh impossible, in my opinion, to give an accurate and comprehensive answer to your title question: What's the meaning of いいじゃん.
Just to sample a few usages of いいじゃん:
If you are sad, cry (literally: isn't it good if you cry)
Someone asked me out!
Nice! Just go have fun.
Here, you should note the 別に which imparts dismissive undertones especially when used in conjunction with いいじゃん. The usage is 別にいいじゃん or いいじゃん別に, and the general idea is unconcern, or impatience, or sometimes even flippancy. "I don't care." but not so explicit. Literally it's like "It's okay, isn't it?" Depending on the context, it can go anywhere between "alright!" "I know!" to "I don't give a damn!" Here I think a simple "Alright!" or "Okay!" would do.
Alright! By the way what did you need to find me for?