You do not have to set up i-mode on your handset. Every i-mode handset has both Pay Monthly and Prepay settings pre-installed. To access i-mode all you have to do is press the key with the yellow "i" on it.
If you buy/upgrade to a Pay Monthly handset then the Pay Monthly settings will be set as default and likewise if you buy/upgrade to a Prepay handset then the Prepay settings will be set as default.
If you are having problems accessing i-mode just double check that you are using the settings that are appropriate to you by: