Thanks for your help, everyone. It turns out the SPSS license I have through my job doesn't let me use custom tables (that costs extra, I guess), but recoding the string variable to a new numeric variable and adding in the original variable's categories as value labels worked. For anyone who has this problem in the future, this is how I ended up doing it:
missing values variable_name ('').
recode variable_name
("Extremely confident" = 1)
("Confident" = 2)
("Neither confident nor unconfident" = 3)
("Unconfident" = 4)
("Extremely unconfident" = 5)
(convert) into variable_name_numeric.
value labels variable_name_numeric
1 "Extremely confident"
2 "Confident"
3 "Neither confident nor unconfident"
4 "Unconfident"
5 "Extremely unconfident".
sort cases by variable_name_numeric(A).
frequencies variable_name_numeric.
execute.