First you have to specify which ArtNet nodes you want to communicate with.
In Startup Configuration go to Manage Devices:

Find DMX Out / ArtNet.

Select the network adapter you want to communicate through.
Then click Detect.

All ArtNet nodes found on your LAN will be listed.

Select the ones you want to communicate with and click Add.
The nodes are now defined as DMX output devices in Aximmetry.

Click OK, Aximmetry will restart.

In your compound you can use the DMX * Out modules to send control to a selected device.

Here you also have to choose a specific Channel within a Net/Subnet/Universe.

You have to send scalar values between 0 and 1 into the Value property.

By default these are converted into a 8-bit value (0 - 255) and sent to the specified DMX channel.
But some devices expect 16-bit values (0 - 65535) layed out into two consecutive DMX channels. For these select a 16-bit Mode, either Big or Little Endian as expected by the device.

By using DMX Array Output module you can also describe a structure of several input data occupying a number of consecutive channels. Each data can be either 8bit or 16bit:

In this case Channel designates the first channel where the data structure begins.

There is also a helper module for sending RGB color data with an optional W component. Components can be either 8 or 16bit: