Enter NULL Value In MySQL Query Browser
It took me literally minutes to find this out, but here's how to manually enter a NULL value in MySQL Query Browser.
There doesn't seem to be a key combination to enter a NULL in the data directly like you can by pressing Ctrl+0 in SQL server. Instead you have to enter edit mode by pressing Edit under the grid, then right click on the cell you want to enter a NULL and select Clear Field Content. A bit of a fiddle as you can't quickly work down a load of rows entering NULL quickly with keypresses. But I guess you should use a SQL statement for that anyway...
Hope that helped you.