That app is written using the Django framework so it stores its dashboards using Django tags and html rather than xml. That is why you are getting that error because there is indeed no xml.
However, if you are familiar enough with html/js you can pick what you want out of the Django app and put it in your own dashboards. You will have to utilize html rather than simple or advanced xml to allow you to paste html in. If you designed your dashboard in xml you can still convert it to html right from the UI.
The aforementioned app's html can be found in the following directory:
(where on Linux $SPLUNK_HOME$ = /opt/splunk/)
if you look inside that dir, inside ad/ and windows/ are some html files containing the prebuilt dashboards.
Once you have created an HTML dashboard using the Search and Reporting app, while viewing that dashboard you should be able to click "edit" in the upper right hand corner and select "edit source". Here is where you will paste HTML and JS code for a dashboard. You can find the code for the splunk app for windows infrastructure dashboards in the directory mentioned in my first post: $SPLUNK_HOME$/etc/apps/splunk_app_windows_infrastructure/django/splunk_app_windows/infrastructure/templates/
For example if you want to copy from the Computer Audit dashboard in splunk_app_windows_infrastructure you would go into $SPLUNK_HOME$/etc/apps/splunk_app_windows_infrastructure/django/splunk_app_windows/infrastructure/templates/ad/audit_computer.html which looks like this
inside the content block is html for displaying the panel and inside the js block is the script that renders the panel. You will need to be able to place these snippets into the proper place in your dashboard which is why being familiar with HTML/JS is required for this.
Also be sure that in $SPLUNK_HOME$/etc/apps/splunk_app_windows_infrastructure/metadata, you have a stanza that looks like this:
# This example would make all of the objects in an app globally accessible to
# all apps
as it states that will allow the objects from the app to be available in other apps such as Search & Reporting as you desire.
Alternatively it may be easier to just make your own dashboard that has the same functionality as the dashboard you are trying to copy.