Why can I not access the cyanogenmod zip on my sd card?
You most probably have an unpartitioned sdcard, clockworkmod however assumes that is partitioned and wants to mount the first partition (/dev/block/mmcblk0p1).

You can fix this by repartitioning your sdcard so that it has a mountable fat-formatted first volume and then retry.

If you don't mind using the commandline, boot to recovery and connect via adb shell to it, then mount your sdcard manually: mount /dev/block/mmcblk0 /sdcard and retry.

Technical explanation:

cat /proc/partitions lists:

/dev/block/mmcblk0 is your FAT formatted volume (no more partitions at all)

should be:
/dev/block/mmcblk0 (whole device)
/dev/block/mmcblk0p1 (1st partition, FAT formatted)
/dev/block/mmcblk0p2 (sd-ext partition, only for those that use SD2ext and such)