The 'Manual Installation' instructions appear to be incomplete (for a full install).
The two files are used to 'update' an existing install. A 'full install' has 3-4 more steps.
Specific to ASUS Nexus 7 3G
The Nexus 7 3G model is not yet supported, but can use the standard Nexus 7 ('grouper') build.
-d grouper needs to be present.
Try using the provided tool, as described on the Wiki.
phablet-flash -d grouper -b (full install, current) or
phablet-flash -d grouper -b --pending (full install, untested daily)
From there, you can probably create the extra manual steps.
To download ALL files first (without a device attached) use:
phablet-flash -d grouper -b -D (current)
phablet-flash -d grouper -b -D --pending (untested daily)
The ASUS Nexus 7 3G (and in particular the 32 GB model) are somewhat different from the base model of the Nexus 7 ('grouper'). They have support for GSM 3G/4G and other low-level firmware changes.
Before Ubuntu Touch was 'flipped' (still ran Ubuntu in a container inside Android), this did not provide an insurmountable problem. However, the additional features not found in grouper were also not available.
Now with the 'flipped' design, and with current changes to the underlying Ubuntu Touch OS and UI, only the base model Nexus 7 (grouper) is currently supported.
Latest two images will not install on Nexus7 (32GB + 3G), but older were Ok
The latest two (newest) daily images will not install on my Nexus 7, 32GB + 3G. The older versions work fine. Background looks like coloured snow. ..
Nexus 7 3G  problem with ubuntu touch
Nexus 7 3G is not equivalent to grouper.
Nexus 7 is a commercial name that hides a lot of different hardware configurations in it.
If the hardware codename is tilapia [2012, 3G] or razor  the device will not work without modifications or what people call ports, please refer to https://wiki.ubuntu.com/Touch/Devices
and more technical:
Ubuntu uninstallable on 32GB 3G Nexus 7
Nexus7 variants without 3G support map their user data partition (UDA) to mmcblk0p9, where variants with 3G support map UDA to mmcblk0p10 ..