First, full installation of OSGeo4W:
http://trac.osgeo.org/osgeo4w/
Use the binary package(#1 under step 2).
Second, fetch the Sextante plugin for QGIS, and add it.
You should now be able to access the GRASS modules(I have currently 154 geoalgorithms).
Source: I used this over the weekend to perform various raster analysis operations, most of which using GRASS-specific tools, all done within QGIS.
Hope this helps.
if you look at the help tab on the processing window, you should (hopefully) see a small web page with help for the algorithm (it'll just take you to the page you linked to in your question, but can save you a trip over to a browser window)
I seem to have a slightly different layout to you, you may have a slightly different version of QGIS and/or Grass to me.
I see the following summary:-
v.net.distance [-gl] input=name output=name [arc_layer=string] [arc_type=string[,string,...]] [node_layer=string] [from_layer=string] [from_cats=range] [from_where=sql_query] [to_layer=string] [to_type=string[,string,...]] [to_cats=range] [to_where=sql_query] [arc_column=name] [arc_backward_column=name] [node_column=name] [--overwrite] [--help] [--verbose] [--quiet] [--ui]
things in square brackets are optional. Ellipsis (...) means things can be repeated
Description files
Somewhere on your system there is a text file which QGIS uses to set up the processing dialog. It'll be called v.net.distance.txt.
If you can't find it in your file system, the Grass 7 description files are here on Github. But those may be more up-to-date than what you have installed.
I don't recommend editing this, but it shows the types, defaults and options for each parameter. (For me, on ubuntu, I'd need to be administrator to edit the file, but not to read it).
In my system this looks like this:-
v.net.distance
Computes shortest distance via the network between the given sets of features.
Vector (v.*)
ParameterVector|input|Input vector line layer (network)|1|False
ParameterVector|from_points|Input vector point layer (from)|0|False
ParameterVector|to_points|Input vector point layer (to)|0|False
ParameterNumber|threshold|Threshold for connecting centers to the network (in map unit)|0.0|None|50.0|False
*ParameterSelection|arc_type|Arc type|line;boundary;line,boundary|2
*ParameterString|from_cats|From Category values||False|True
*ParameterString|from_where|From WHERE conditions of SQL statement without 'where' keyword||True|True
*ParameterSelection|to_type|To feature type|point;line;boundary|0
*ParameterString|to_cats|To Category values||False|True
*ParameterString|to_where|To WHERE conditions of SQL statement without 'where' keyword||True|True
*ParameterTableField|arc_column|Arc forward/both direction(s) cost column (number)|input|0|True
*ParameterTableField|arc_backward_column|Arc backward direction cost column (number)|input|0|True
*ParameterTableField|node_column|Node cost column (number)|from_points|0|True
*ParameterBoolean|-g|Use geodesic calculation for longitude-latitude locations|False|True
OutputVector|output|Network_Distance
The ones starting with "*" go into the advanced section, as far as I can see. The bit between the first and second pipe characters is the name of the GRASS parameter, which you can look up in the help. The bit in the next field is the label which appears in the dialog.
This trick can be useful if you find the label ambiguous or unclear.
As Joseph says, some of these are actually routed to v.in.ogr (or v.out.ogr) instead, but these are clearly labelled.
Best Answer
I managed to get it working. Let me elaborate. I had to add an aditional nlayer to the network. I did this with the v.net tool. Thus in the end i had 3 layers in the network. 1. the lines being the network itself. 2. the nodes at the end of each line segement. 3. the point i wanted to use as center of the isochrone. (you can check this via the browser by opening the network database) Then with the identify feature I was able to identify the 'cat' of the node i wanted to use as center. This 'cat' (in this case 1, which is guess means it is the first node layer other than the nodes needed for the calculations) i used as cat input in the v.net.iso gui. This gave me the results i wanted.