On Tuesday, I didn't have time for Glee, so I had to fast-forward through the episode in search of Emma until I could watch it in full. So I missed this.
I've never seen this dress before. It looks like a red polka-dot jacquard fabric with a drop waist and a pointed v-neck collar.
She's accessorized with a (probably) vintage brooch, and this bracelet:
Kate Spade Garden Party Link Bracelet - no longer available
I'll let you know when I find this dress!