I need a MSN Messenger app, but I need one that does not require a BB plan. I have tried RIM's Y! messenger, and Jivetalk and both refuse to work because I donthave a BB plan (I have a data plan; which gives me unlimited data X-fer on both handset and when using it as a modem)

If your looking to use a BlackBerry on the cheap then you bought the wrong device to begin with. BlackBerry's communicate through RIM's secure servers and only RIM's servers, without said BlackBerry Data Plan you'll have no data functionality, email, web, IM etc. just voice and SMS, end of story and no there's no work around. Please use the search feature and search for IM or MSN.