I have MS Office 2007 on a Microsoft Vista, and I am trying to install the PLUSTOOLS Template on it, but I don’t know where exactly I have to locate the “PlusTools.dot” file.
You can put PlusTools.dot (almost) wherever you want. The "normal" (= expected by Word by default) is:
Note that AppData is a hidden folder, so you may have to make hidden folders visible in Windows in order to be able to navigate to the Templates folder.
I tried to follow the instractions given on Wordfast.net, where it says I have to go to “Word Options”, then “Add-ins” but I can’t find the “Add-ins” option in my MS Office.
These instructions are for Word 2003 and older, which are the supported versions for PlusTools. Word 2007 and newer are not supposed to be supported (it's stated on the download page if you read carefully), but you should usually be able to make it work nevertheless.
In Word 2007, click on the ugly Office button in the upper left corner, select Options, Add-ins, Manage = Word Add-ins, Go, Add... and point to the folder where you copied PlusTools.dot.
You only need to add the PlusTools template once. Thereafter, whenever you want to use PlusTools, go to the same place (Options, Add-ins, Manage = Word Add-ins, Go) and tick the PlusTools template to select it.
I assume you want to use PlusTools for aligning documents. If so, you may want to consider a more "modern" aligner, eg. either of the following: 1) the Wordfast Online Aligner included in Wordfast Anywhere, or 2) LF Aligner, a free tool available at http://sourceforge.net/projects/aligner/
You can find a short video (made by me) about LF ALigner here:
There is a video about the Wordfast Online Aligner on the Wordfast wiki: