and you are
-1 : have the keyboard slider open.
-2 : have the cable at the ready plugged into the pc but not the device yet.
-3 : you are holding down on the "u" key on the keyboard and then proceeding with plugging in ...
-4 : you "should" have the usb icon in the upper right of your device screen

If the above has been done correctly...
then you have therefore done everything right up to this point and eve if at this point the device isn't recognized by the os...
I would hazard it is an issue with the os (did you try the old buntu distro cd that pichlo specifically mentioned or are you using the latest one with Unity?)...
have you tried flashing on a different computers even...
(to rule out the possibility that your problem is n900 device related?...)
in specific as Pichlo has mentioned (that he had to use a variety of computers before finding one that wanted to play nice...the same issue has been noted with other people....for myself I haven't had a prob flashing with the few computer brands I favour as much as had a problem with specific os's in my time...) there can be singularly persnickety os's and computer brands which can make things feel that nothing is going right...

for myself dell, toshiba and ibm and ibm/lenovo and lenovo as brands (pre-2013 ...don't know how the latest are preforming..) always worked for me.
32bit debian or buntu (even the latest lubuntu 32bit works fine..just did a test yesterday to see how the latest would perform...), win xp (win 7 had to be tweaked a bit)
always seemed to perform without a hitch...