Where is the Vostro 3350's ExpressCard Slot

Isn't it on the right side of the laptop "in front" of the DVD drive?

No, it isn't. 

For the older 3300 model: yes

The 3350 doesn't have one (according the the manual).  The specs listed are wrong apparently -- the 3400/3450 (14" chassis) does, though.

http://support.dell.com/support/edocs/systems/vos3350/en/SFITS/sfits_en.pdf

Hm, but I know I shop where the 3350 is offered with UMTS card !

And I can't believe that all shop offers and product anouncements are wrong :emotion-42:

Hence now I guess it is somewhere inside the laptop's case and you have to open it !???

Not possible - Express Cards are external devices, by design.  WWAN cards can however be PCI Express -- those DO go inside the sytem.

Thank you, now  I see.

I plan to buy those WWAN card. I opened the case:

Where is the slot for those WWAN cards ?

Remove the keyboard.  The WWAN socket should be at the top left of the system, just under the display panel.

Ok , under the keyboard. as described in the old 3300 manual  !

Thanx.

express card slot in dell vostro 3550 is on right side, beside dvd drive.

now u u can find.