You don't actually have to have a piece of obsidian in your inventory. Go to the portal (Marked as
3 here) and you'll get a 'dialogue' option which says something like "pick up an obsidian stone from the ground and cut your tongue with it, thus activating the portal".
Also, seeing as this question is an extention of your previous one, I'm merging the threads so that anyone searching for the question will only need to look through one thread. Call it good record keeping if you will

Speaking of searching, if your question still isn't answered you might also want to read through
older threads addressing the same question. As a matter of fact, it is usually a good idea to make a
search before you post a question, seeing as your question might already be answered.
edit: Doing homework while simultaniously posting results in other people posting nearly identical replies to mine, while I'm writing mine, ten minutes before I post mine. I need to start multitasking more efficiently
