To unlock those secret missions you have to do the mission challenge AND the secret challenge to open the new areas.

Mission	How to unlock
Mission C26	Complete the whole Course on mission C16
Mission C31	Complete the whole Course on mission C25
Mission D26	Do 220 or more missed steps on mission D29
Mission X1	Do 40 or more 'GREATs' on mission A5 or get a rank of 'AAA' on mission A6
Mission X10	Get 'OK' on all Freeze Arrows on mission C29
Mission X11	Do a MAX Combo of exactely 77 and less than 4 misses on mission C20
Mission X12	Do a Full Combo and get 'OK' on all Freeze Arrows on mission C19
Mission X13	Get a rank of 'A' or better on mission X9
Mission X14	Get 8,500,000 or more points on mission C35
Mission X16	Get a rank of 'A' or better on mission D14
Mission X18	Get less than 4 'GOODs' on mission D16
Mission X19	Do a Full Combo on mission D19
Mission X2	Get a rank of 'A' or better on mission B5
Mission X20	Get 7,000,000 or less points on mission X16
Mission X21	Do 80 or more 'GOODs' on mission X20
Mission X22	Get a rank of 'AA' or better on mission D18 or do a Full Combo on mission X19
Mission X23	Do 8,000,000 or more points on mission D26
Mission X3	Get a rank of exactely 'C' on mission B24
Mission X4	Do exactely 100 missed steps on mission B26
Mission X5	Complete the whole course on mission B37
Mission X6	Do 70 or more 'PERFECTs' on mission B51
Mission X7	Do a Full Combo on mission C5
Mission X8	Do 80 'GREATs' or more on mission C11
Mission X9	Do a MAX COMBO of exactely 10 on mission X8