If the string value does not display a 0 before the single digit month values, we will also need to check for the length of the string value first.

If the string is 6 characters long, it is a single digit month.
If the string is 7 characters long, it is a double digit month.

Create the following formula:

if Length({CnCrdtCrd_1.CnCrdtCrd_1_Expires}) = 6 then
     Date(ToNumber(Right({CnCrdtCrd_1.CnCrdtCrd_1_Expires}, 4)), ToNumber(Left({CnCrdtCrd_1.CnCrdtCrd_1_Expires}, 1)), 01)
else if Length({CnCrdtCrd_1.CnCrdtCrd_1_Expires}) = 7 then
     Date(ToNumber(Right({CnCrdtCrd_1.CnCrdtCrd_1_Expires}, 4)), ToNumber(Left({CnCrdtCrd_1.CnCrdtCrd_1_Expires}, 2)), 01)