I opened a margin position but there is no change to my currency balances?

Kraken employs FX-style margin trading. This means that margin trading does not result in any exchange of currency in your account. For instance, if you open a long position with a leveraged buy order, you will not receive bitcoins in your account. If you open a short position with a leveraged sell order, bitcoins will not be deducted from your account. Instead, all profit or losses from the position are unrealized until you close the position, at which point adjustments to your currency balances will be made to account for the profit or loss. Adjustments to your currency balances are usually made in the base currency of the pair you are trading (i.e. XBT in the pair XBT/EUR).