Must in Japanese has a special construction. Instead of giving an "order" positively, they use a negative form.
To make you understand I try to use an example. If I want to say "tonight I have to go to work" in Japanese I will say "If tonight I didn't go to work, it wouldn't be good". I am not sure about English, but it sounds good to my ears coz even in Italian (my language) we use "can't go" like they use it in Japanese to mean "it is not good". Apart from ikemasen (I don't know if you can read Japanese from that computer so I use the hated romaji) it is used also the verb naru (here negative narimasen). Mind though that naru doesn't mean only change. In this case it has more a meaning of "suit".
As per need, I have mostly found it written in hiragana and probably that's why it confuses you. But as Duo said, this iru has nothing to do with the iru=exist. Two different words.
EDIT: Hopefully you haven't read my answer yet
I just noticed you wrote (shi)nakereba...
Well, nakereba is the conditional form of the negative particle NAI. Thus in that case is put after the verb suru (which in negative becomes shinai). If you use another verb, leave out the shi and use the negative root of the verb. So in my example above it would have been (to work=hataraku): hataraka-nakerebaikemasen.