PLCUX is traded only in PLC Ultima pair, so before you buy PLCUX, you need to buy PLCU coins. You can find detailed instructions on how to buy at this link. 

Now, that you have PLCU coins on your Coinsbit balance, go to the Trading section of the exchange and enter PLCUX in the Search section in the left menu. This will bring up a pair with PLCU, click on it. Or follow this link:

To buy at market price select Market:

In the Amount column, enter the PLCUX amount you want to buy:

Then click BUY.  

You can also buy PLCUX under a limit order — an order with a specified buy or sell price. To do this, select Limit and specify the price you want to buy PLCUX for and the number of Coins you want to buy. Then click the BUY button. 

It's done! You have bought PLCUX Coins. To replenish your farm, follow these instructions.