If you already have a channel store account, the only "code" you need is the one for the specific "private channel" you want to add. You then log into your ROKU account (from your PC), click on the "Add Private Channel" link, and then enter the code for the private channel you want to add. When you next enter and exit the "channel store" on your ROKU player, the private channel you added (from the web site) should be present (because entering and exiting the player's channel store, causes the ROKU to sync with your account info on the ROKU web site).
As to how to get the channel codes for the specific private channels you might be interested in, that varies with the channels. By their vary nature "private channels" are not setup for general distribution (hence why they are called "private channels"). The "public channels" all can be added directly from the ROKU box via the "channel store". However, the "private channels" require you to get the hidden "private" code to enable the channel (from the channel developer), and then enter it into your ROKU account on the web (in order to enable a "private", not publicly released, channel on the box).
However, in the case of private channels actively discussed on this forum, most of those "channel codes" are posted (by the developer of the channel) in forum messages discussing the channel. So in that case, you just read the forum posts about the channel, and see if one of the posts mentions that the channel code for the channel is xxxxx. If so, that's all you need. Because once you get the channel code from the developer (even if/when the process is as easy as reading a public forum post about the channel, that mentions the code to use), you just use that code (via the procedure mentioned above) to enable that channel on your ROKU.