Well, if you are still using v0.6.3, you will not have the blank cell checkbox in the preview. Download v7 again, and retry.
Also, you must use the preview from the trace, not with the ~ key. Enable the automatic preview mode with the play checkbox to the right of the trace menus, and run the trace.
In trace mode, each cell is played one at a time. In the preview window, you can enable the "Blank this cell" checkbox to blank the current cell.
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