according to the userguides :
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To clear one bookmark
Right-click and select Toggle Bookmark | Bookmark# (CTRL+SHIFT+# where # is a previously defined
bookmark between 0 and 9).
To jump back to a bookmark
Right-click and select Go to Bookmark (CTRL+# where # is a previously defined bookmark between 0 and 9).
Note: The # must be called from the number row on the keyboard. Using the Number pad will not call the bookmark.
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that's a pitty ! 
Why not enable the Number pad? that would be great and would make coding easier and more efficient.
I'm using Toad 13.1