I assume you entered the line (res.job_abilities:with('name',"Flash")) into the chatlog or console? If so, that will fail, but otherwise that works. However, Flash is a spell, not a JA, so this will obviously be nil.
//eval print(res.spells:with('name', 'Flash')
This will print out the table entry and its index (spell ID in this case). However, this will not work:
//eval print(res.spells:with('name', "Flash")
The reason for that is the double quote. Windower basically removes double quotes, because they are used to group arguments together. You either need to use single quotes like above or escape them like so:
//eval print(res.spells:with('name', \"Flash\")
But again, this is only an issue for the chatlog or console. In a Lua file both single and double quotes will work just the same.