Do I have access to the private key for an Instawallet? It is just numbers and letters? I have transferred my bitcoin address and want to use the key so I can move my coins from this wallet to another? As you may be aware there site is down but can I set this up on another wallet so I can still get my coins?
No, Instawallet is a hosted (shared) EWallet.  

They do not provide to you the private key for the deposit addresses associated with your Instawallet.

When you request a withdrawal, Instawallet processes that request and makes the transaction.  There is no way for you to create a withdrawal transaction yourself for any funds in an Instawallet.