If an app says some additional fees are required how do you pay those fees. Went to the channelstore and nothing comes up. Get the app and nothing.
If you added the app and it didn't ask for any payment when you added it, it will probably show what fees apply to what when you launch the app. They're paid via the payment method attached to your Roku account. Many apps are free to add, but may have in-app purchases available. I highly recommend adding a PIN to your account at https://my.roku.com so you won't inadvertently purchase something without realizing it.
More information is needed. What is the app in question, what model Roku device do you have, and where are you located (country)?
Many apps/channels can be installed for free, but you must subscribe (pay a fee) to use the app. For example, you can install Netflix, but you can't watch anything without paying for a subscription.
If you added the app and it didn't ask for any payment when you added it, it will probably show what fees apply to what when you launch the app. They're paid via the payment method attached to your Roku account. Many apps are free to add, but may have in-app purchases available. I highly recommend adding a PIN to your account at https://my.roku.com so you won't inadvertently purchase something without realizing it.