All forums have two buttons to look out for:
+ Post New Thread
And
+ Reply to Thread
So, you first Pick the forum you want to post in, say Refrigerants (http://www.refrigeration-engineer.co...7-Refrigerants)
Then, once you have surfed there, you will see a lot of different thread. To post a NEW thread, you click the +Post New Thread button (in green and white on the left)
If you wanted to look at a specific thread, then say pick? about hydrocarbon gas (http://www.refrigeration-engineer.co...ydrocarbon-gas)
Then once you have surfed there, if you want to reply, click the +Reply to Thread button (in green and white on the left) or click in the quick reply box below that and start typing.
Simples