I have often heard the phrase: “You couldn’t organize a fuck in a whore house.”
While watching Hercules Returns one of the characters says, “You couldn’t organize a fart in a curry eating contest.”
Dopers, I challenge you to use this template to create new ways to express incompetence. 
Enjoy!
Good luck, but insult threads don’t generally last a real long time.
Subscribe (if it still exists) to Maladicta, dedicated to the art of vilifying one’s fellow man.
And I couldn’t organize a tea party in a dollhouse. Or a spark in a tinderbox. Or a bonfire in a lumberyard. Or a conference call at an AMWAY convention. Or an orgy at a whorehouse. Or, well, better go.
The standard one I hear is
You couldn’t organise a piss up in a brewery
How about this:
“You couldn’t organize a hunger strike at a fashion show.”
You couldn’t organize a Tom Cruise Cheer at a Scientology Convention
You couldn’t organize a(n) OCD patient in a mental ward.
‘Couldn’t organize a gang-bang in a nunnery’.
Not my invention, I hasten to add.
Alternately: You couldn’t organize a coke line at a fashion show
You couldn’t organise a screaming argument in a Straight Dope obesity thread. 
You couldn’t organize a shower in a rainstorm.
You can’t find your ass with both hands and radar?
Three that I’ve read attributed to LBJ: “He couldn’t organize a two-car parade,” “He doesn’t know chickenshit from chicken salad,” and “He couldn’t pour piss out of a boot with the instructions printed on the heel.”
I’d like to join in, but … I can’t fill in a blank even when there are some. 
I may or may not find my ass, but no way am I getting Gary Burghoff involved.
This has been on my mind of several days. I don’t have the skill to fill the blanks on my own, so I threw this tibit to the SDMB - gobs of clever people. 
Damn my vivid imagination. Must. . . get. . . image. . . out of head!
You couldn’t organize a salad in the produce section.
Are ya sure? He’s good at finding the dumper (so I hear).
You couldn’t organize a circle-jerk in a steam room.
Graphic…crude…yep, it’s got everything.
You couldn’t organise a draw in a one legged ass-kicking competition.
You couldn’t organize a Get-Out-of-Jail-Free lottery on Death Row.