Calling ports in C on my microcontroller
You don't "call" an IO port, you manipulate the registers that control it. You should have the LPC13xx.h file from NXP, which defines C data structures that you can use to access the ports. After including this file you can control the port pins in C with something like:

LPC_GPIO3-&gt;DIR  |= (1 &lt;&lt; 4);  // make P3.4 an output pin
LPC_GPIO3-&gt;DATA |= (1 &lt;&lt; 4);  // set P3.4 high
LPC_GPIO3-&gt;DATA &amp;= ~(1 &lt;&lt; 4); // set P3.4 low


Of course you need to add a delay loop of some kind if you want to see the LED turn on and off.