Upon clicking the "Download" button, you will be presented with the option to download two files; you need to download them both. Head into SimCity 4 and create a new region; in this example, the region will be named "Eaton", then quit back to desktop. Copy and paste both of the downloaded images into Documents/SimCity 4/Regions/Eaton (replace Eaton with the name of the region you just created). Open up SimCity 4 again, and press Ctrl + Alt + Shift + R. This will make a new window appear. Within the new region, you need to navigate to your Documents/SimCity 4/Regions/Eaton folder, selecting the greyscale image from the download entitled "Eaton_Jhipolito.jpg", and then click OK. The game will then render the region.
If, for some reason, one of the above steps does not work, try using SC4Mapper to install your maps. Once installed, all you have to do is click "Create Region", select "Greyscale Image" format, and in the next window which opens, select the path to the downloaded "Eaton_Jhipolito.jpg" and also the downloaded "config.bmp" files.
Once you've subscribed to an item in the Steam workshop, you'll need to enable it in your game. To do this, click "Content Manager" on the ingame main menu, navigate to the "Mods" tab and then check the checkbox labelled "On/Off"
Any Maxis Highway retexture mods (such as the HRS and the Asphalt Highway Textures mod) will need to be placed in a subdirectory of your Plugins folder. This should replace the textures of the elevated highway too. The folder should be prefixed with something like "zz__" to ensure it loads after the Network Addon Mod.
For example, Documents/SimCity 4/Plugins/zz__Retextures/file.dat
Try putting the folder into the z___NAM folder in your plugins.
If this fails, this mod is included in the NAM itself. Run a custom install, and under "Road Textures and Markings", select "Asphalt Textures for Maxis Highways"
About the concrete textures, (these are very early textures, the final things should look much nicer), but I've hardly done anything since. I'm hoping to start working on them again in the future, but as Maarten says, there are a lot of textures which need to be reworked.
Apologies for the late reply, this is due to the plugin not being installed in a subfolder of Plugins, eg Plugins/zzz_Highway_Restyling_Mod/file.dat. Make sure it loads last to avoid conflict with any other highway retexture mods.
Yup, those textures are something I've been meaning to patch, however I've not got round to doing it. I don't think I'll ever be able to get them to be perfect though, due to the way the Maxis Highway's textures are "assigned". I'll see what I can do