Hey mate. So hive is a bit funky with the way things are set up but ill try to point you in the right direction.
Your splinterlands username is your hive account name(wallet).
You need to request your hive keys from the splinterlands game. This can be done via the ingame drop down menu where the settings are. These will be emailed to you, there will be 4 keys. Dont lose them! Write them down!
The 4 keys are
Master / Owner (This allows you to reset all keys / passwords - never share this one)
Active (This allows for transactions, required for buying/renting cards)
-Posting (This allows for posting / commenting and is also the lowest key required to play the game)
-Memo (this is used to send or look at encrypted msg)
- Install the HIVE keychain ext for your desktop browser
Once you have set this up, you will now be able to access everything on the hive blockchain as splinterlands/peakd etc are basically front ends running on the blockchain using the same data.