USS Sea Tiger,
If you don't have NYGM yet, you just need to download and install over a clean copy of SH3 patched to 1.4, the following:
First, the main NYGM install:
http://files.filefront.com/NYGM+Tonn.../fileinfo.html
Second, the latest patch (3.2):
http://files.filefront.com/NYGM3+27z.../fileinfo.html
Optionally (Not required but nice to have):
Hi-res U-Boats:
http://files.filefront.com/NYGM+HiRe.../fileinfo.html
IABL Merchants:
http://files.filefront.com/IABLShips.../fileinfo.html
Once you have installed that, you just start JGSME and enable the mod that includes the Type XXIII, which is the one called "FuMB37XXIIINYGM"
Happy hunting