domAsset::domUnit is trying to give the unit for measurement unit purposes. (for example, you want to specify translate of 1.0f unit in your document means 1 km).
For most of the time, you won’t need to use it.
To create an exporter to export “a mesh material with texture” you need to export at least 3 groups of things
Here is a simple code to create 5 <image> elements from scratch.
domCOLLADA * c = dae->getRoot(file_name);
int num_of_images = 5; // let's say you have 5 texture files
domLibrary_images * li = (domLibrary_images*) c->add(COLLADA_ELEMENT_LIBRARY_IMAGES);
for(int i=0; i<num_of_images; i++)
domImage * image = (domImage*) li->add(COLLADA_ELEMENT_IMAGE); //add a new image to library
//image->setId(YOUR_UNIQUE_IMAGE_ID_FOR_IMAGE_i); //add more attribute to <image>
domImage::domInit_from * init_from = (domImage::domInit_from*) image->add(COLLADA_ELEMENT_INIT_FROM);
//init_from->setValue(YOUR_IMAGE_PATH_NAME_FOR_IMAGE_i); //add more element or attribute in <init_from>
After you set up your <material>, <effect>, and <image>, you have to make sure that they are being referenced correctly. A <material> links to a <effect> that could like to one or more <image>.
This is not easy for someone new to COLLADA. I suggest you to export “a mesh material without texture” first.
You will also need to “bind your mesh with a material in your scene” before you can actually see your result in a rendering scene or reference viewer.
Let me know if this is what you are looking for.