There's no way you can add any data package to the 10c/min plan if you're using an iphone/smartphone. WI-FI is your only option.
The 50MB data package can ONLY be added to the $25 plan, There's no data allotment included in the $25 plan. So if you want to use data, you have to buy the 50MB package.
You need to pay the $5=50MB every month, any balance remaining will rollover if you add before package expires. (packages are independent to the plan and their 30 day period will only start when you add the package)