The way I installed the apk without using xapk installer is as follows:

1) used 7zip to open the TomTom NDS MOD 1.8.15.1.xapk and extracted the following;
a) Android folder
b) com.tomtom.gplay.navapp.nds.apk

2) copied the com.tomtom.gplay.navapp.nds folder from within the extracted android/obb folder to the internal memory of the phone at android/obb

3) copied the com.tomtom.gplay.navapp.nds.apk to sdcard of the phone and installed the apk.

All is working great.