In general private blockchains aren't secured by proof-of-work, so difficulty is not really relevant.
If you want your MultiChain blockchain to be secured by proof-of-work, set anyone-can-mine=true in the blockchain parameters and the target-adjust-freq to something sensible. The difficulty will then automatically adjust in the same way as bitcoin's, to stay around the target-block-time.
Note also that the pow-minimum-bits parameter can be used to adjust the difficult of building a block, even in a blockchain which is secured by mining diversity between permissioned miners, though I can think of any good reason why you would want to do this.
