I'll be happy to help, though this isn't the correct subforum to ask it in....
Go to User CP in the upper left hand corner.
On the left hand side will be a box with an assortment of tools; click "edit signature".
In another window, go to your diary thread on the page you want it to link to. Copy the URL in the address bar (at the top of the screen; the one that says http).
Now, back to the "edit signature" thing. In the box provided, click on the link icon (the world with paperclip things on it)
Paste in the URL you just copied.
Press okay.
Type in the name you want it to say (My Diary or whatever).
If you need any more assistance, let me know and I'll be happy to help.