What attracts CO2 into the Calvin Cycle?

CO2 diffuses down its concentration gradient to the point it is removed from the gradient by the action of the enzyme RuBiCO. CO2 and RuBP are substrates for the enzyme and they are attached to each other in the reaction effectively removing CO2 and creating the gradient.

Rubisco is the enzyme/protein that will catalyze/speed up carbon fixation process with CO2 joining with RuBP (the two substrates that will enter the "active site" of the Rubisco enzyme and form an "induced fit"). Enzyme/Rubisco will bring the CO2 and RuBP into close proximity (one way an enzyme can catalyze a reaction). Once they join, they will form an unstable 6 carbon molecule which will immediately split into two 3 carbon molecules. They will be reduced (gain electrons) by NADH transfering 1 electron and thus becoming NAD+ and phosphoryated by ATP. This will generate 2 G3P per CO2 fixated. One G3P will leave to stroma and form the monomer glucose (or it's derivatives) with other G3P, while the others will gather up to regenerate RuBP. Hope that helps.