How can I get the /opt folder to appear in Finder

There may be an easier way to do this, but I know this will display /opt via TextEdit

TextEdit -> File -> (hold Option key) -> Save As...

Expand your Save As dialog box via the triangle at the end of the "Save As" field (triangle should be pointing up)

On the 3rd line select the Column browsing mode

Click on the folder name popup list and select your system name (should be the name you have in System Preferences -> Sharing -> Computer name)

Now the Magic: Command-Shift-period

Now Click on your boot disk (most likely "Macintosh HD", or similar) and you should see all the hidden files, including "opt"

That folder is hidden for good reason. Why (and what) do you want to paste in there.

You can use the Finder Go, Go to Folder... dialog (just paste in the path), or you can use commands in Terminal (cd, ls, cp, etc.).