I had this issue on the Dell Venue but was never resolved. On an ASUS ZenPad, what is the purpose of the SD card if it can't be accessed? So how does one play movies from the SD card?
4.4 had a change that made all but one folder per app on the sdcard read only, but you could disable that... you should be using 7.1 anyway... why can't you access it? Did you format it as internal storage? If so you are using it, just transparently.
Is this the correct model? https://wiki.cyanogenmod.org/w/P024_Info
Check that it is not formatted for a pc, which usually uses ntfs, whereas android can only read fat32 (also readable by windows, but has 4gb max file size :( and ext4. I also saw people in forums talking about the asus zenpad not supporting exfat, so make sure it's not formatted as that either. To reformat using windows: https://www.sdcard.org/downloads/formatter_4/index.html hope that helps :)