How do you repair a gear oil leak on a 94' VW Jetta?

What is your question?

There is a small Plastic Cap back from the Oil Dipstick that I was told is where the Gear oil goes but when I put the Oil in this tight small hole (closed with a clear plastic cap) is runs straight through to the ground, is this suppose to happen? If so how do I Repair it. I Googled it and it told me it's called a Gear Differential, then Googled how to repair and it said get the Permatex Gear Oil RTV Sealant (3 oz), is my Research Correct and Please tell me what to do. This isn't serious I hope.