Move to the next unread article. You are prompted for a search pattern. The commands at the bottom of each screen in slrn are context sensitive. That means slrn will display commands appropriate to what you are doing at the present time. That is, 's' and 'S' are not the same.
Some commands take a prefix argument.
This means press ESC, then press a digit followed by desired command key sequence. Move to the group you want to subscribe to and then press 's'. Repeat this step for each group. Press the Capital-L key again to display only subscribed groups.