The first stop should be checking if the kernel is at all seeing and recognizing your sd card.
Open a console and type
that will show kernel messages. After you plug in the card, the kernel should immediately say something.
in my example, just plugging in a USB SD card reader:
[179708.322673] usb 4-2: new high speed USB device using ehci_hcd and address 2
[179708.459443] usb 4-2: configuration #1 chosen from 1 choice
[179708.460990] usb 4-2: New USB device found, idVendor=0781, idProduct=a7c1
[179708.460992] usb 4-2: New USB device strings: Mfr=3, Product=4, SerialNumber=2
[179708.460994] usb 4-2: Product: SDDR-113
[179708.460996] usb 4-2: Manufacturer: SanDisk
[179708.460997] usb 4-2: SerialNumber: 000000009412
[179708.618561] Initializing USB Mass Storage driver...
[179708.619036] scsi8 : SCSI emulation for USB Mass Storage devices
[179708.622560] usb-storage: device found at 2
[179708.622560] usb-storage: waiting for device to settle before scanning
[179708.619036] usbcore: registered new interface driver usb-storage
[179708.619036] USB Mass Storage support registered.
[179713.772009] usb-storage: device scan complete
[179713.772009] scsi 8:0:0:0: Direct-Access SanDisk SDDR-113 9412 PQ: 0 ANSI: 0
[179714.006672] sd 8:0:0:0: [sdd] 7744512 512-byte hardware sectors (3965 MB)
[179714.006672] sd 8:0:0:0: [sdd] Write Protect is off
[179714.006672] sd 8:0:0:0: [sdd] Mode Sense: 03 00 00 00
[179714.006672] sd 8:0:0:0: [sdd] Assuming drive cache: write through
[179714.009798] sd 8:0:0:0: [sdd] 7744512 512-byte hardware sectors (3965 MB)
[179714.011794] sd 8:0:0:0: [sdd] Write Protect is off
[179714.011794] sd 8:0:0:0: [sdd] Mode Sense: 03 00 00 00
[179714.011794] sd 8:0:0:0: [sdd] Assuming drive cache: write through
[179714.011794] sdd: sdd1
[179714.011794] sd 8:0:0:0: [sdd] Attached SCSI removable disk
Now from this you can already know a lot. What typ of media it is and how many partitions are on it etc... That is the fun in Linux, you can always see what is going on.
Now at this point Linux should offer you a dialog, like "a new medium has been detected, what do you want to do?" offering to mount it. Pretty much any newer Linux will.
That means it also recognizes the filesystem on it.
Now if you don't see any message in dmesg, your kernel probably does not recognize the reader. OR the reader cannot read SDHC (high capacity) cards, meaning anything bigger then 2 gigs. Try sticking the SD card in a USB reader that you sometimes get free with the cards. It should bear the SDHC logo.
Usually, if your reader is not supported (or any hardware) it works best to try a newer kernel. In Linux, all hardware driver come packaged in the kernel, no installation is usually necessary.
So just popping in a brand new live CD and running that thing without installing it can give you a spanking new kernel to try out. Pop in the card again and see if there is a change.