Are you trying to upload a picture or file ? or are you just wanting to add a link to your post ?
If you are trying to add a link to your post it sort of depends on the browser you are using.
Some will automatically make them a hot link when you past the web address into the reply area where you type out your post.
Other browsers require you to make them hot using the tools here.
So to make a link "hot"
You can copy and paste your link into the message area of your post like this. (pic 1)
Then you need to hold your left mouse button down and drag across the link highlighting it. (pic 2)
Once highlighted hold your mouse over the highlighted text and right mouse click and select copy. (pic 3)
Then left mouse click on the chain link icon in the tools above (pic 4)
In the drop down box the first line is the Link URL box place your mouse there and right click and select paste and then the insert button (pic 5)
Now you should see your link highlighted in the message body. Finish completing your post and hit the submit button (pic 6)