if they are summerboard themes then you need to install summerboard from the system catergory in installer. Themes for customize require customize which is under utilities.
if themes dont show in summerboard when installed then add the following source to installer:
then go back to install and look for the category iPod Touched, in there scroll down to SummerBoard Theme Fix this should then hopefully have the themes showing in the themes section. you will need to re-install this [atch when u download new themes (unless they already show in the themese section after download)
oh yeh SummerBoard will appear on the home screen as SMBPrefs
only if the theme is for the customize program, the categories are split into summerboard anc customize so just go with the summerboard one and you will be set