Wow, double wow and Triple Wow
Wow number 1-
Did you know what kernel was there before flashing a new one?
That may have gotten the ball rolling on your prob there....
If you flashed a different kernel than what you should have...

Double Wow-
The flasher you used...
perhaps for the 770, n800, n810 it works with an OSX host computer....
but that flasher really never was meant for the n900 ....
if memory serves...
Personally I've never tried flashing with a osx host machine so I'm perhaps not one to look to on how well the flashing process works utilizing osx hosts ...

Triple Wow-
That "unable to find device" is really about the flasher and the host communicating improperly with the n900...due to missing drivers on your host and the improper flasher...it couldn't find the device...It's looking for an n770 or n800 or even an n810...not an n900....and the host not having requisite drivers in place to recognize the n900 when it connects doesn't help....But even so....

Triple wow...
Wowwowwow...

Be a wee bit concerned at this point ...
but not worried or horrified or sweating bullets...
I think it should be fixable...

Best suggestion is grab fffff whatever-flasher () and all your pertinent flashing images and files and use a (friend's) linux machine (this flashing tool works grand)
or
grab the nokia internet tab. software update wiz. and use a (friend's) winxp / win7 machine. ...relevant driver to allow win7 see the n900...
or
grab flasher 3.5 (think it still works fine ...haven't tried it in a bit ...6months-1yr...)

Those methods work well...
Just gotta do it right.
Just make sure you pay attention to instructions...
regardless which host type (linux or ms) you use...
Shout out if you hit a wall along the way...
I'm sure one of us will chime in with help...
Do it right and it should put your device back in operation.