[Q] CWM can't mount sd card

hi
I rooted my galaxy s3 GT-I9300 about two days from now (4.1.2)
and I downloaded MOD from this forum
so I went to recovery CWM to install this MOD
I pressed install ZIP then choose ZIP from sd card
but nothing happens only I saw below
E: can't mount /sd card/
so what's the problem ??:o