If you wanto to lend coins, you have to transfer funds from your Poloniex Exchange balance to your Lending balance.
Those are internal balances managed by Poloniex and you can freely move funds from ono to the other instantly and with no cost.

Go to your poloniex account menu, and click Transfer Balances :


Then choose the amount you want to transfer from your Exchange balance (your main balance) to your lending Balance:


And that's it! As soon as PAL detects your lending balance, it will start working on your loans.


Related links:
Transferring To Funding account (BITFINEX)
How it works
Setting up Api Keys