I face a similar problem.
Windows 7 64bit SP1
When Installing npp.7.5.6.Installer.exe (32bit) the txt files get associated to Notepad++, or at least I can create the association from the context menu ‘Open with…’ / ‘Choose default program’.
Wen installing npp.7.5.6.Installer.x64.exe this is not possble: not only the entry for Notepad++ doesn’t appear on the context menu ‘Open with…’ , nor in the dialog opened from ‘Choose default program’,
but even if I browse to '“c:\Program Files\Notepad++\Notepad++.exe” and press ‘Select’ the association is not created.
I use WinPatrol, and maybe that is interfering and causing the problem, but this problem still doesn’t appear with the 32 bit version of Notepad++.