SPSS is not recognizing my grouping variables?

I'm trying to perform a Mann-Whitney U Test using the Two-Independent Samples Tests dialog box but it does not show any of my nominal variables to select as Grouping Variable.

I've searched everywhere for an answer as to why these variables are not showing up...
I've added value labels and everything else suggested that I came across on the internet but still no solution. They are Sting values, Nominal, not sure what else you might need to know to help.

I have SPSS 19 and I'm running Windows 7 64-Bit.

Thanks in Advance
You can't use string variables in analyses. You need to recode these variables into numeric values. For example, if gender was your grouping variable you'd assign '1' to females and '2' to males.