Here is a simple explanation for how to load a linetype. I decided to demonstrate how to do so for an appropriate layer but a line type can be loaded from the “Properties” panel as well.
Here’s how:
- In the Layer Dialog box, under the “Linetype” column, click on the linetype name that is set to your layer. In the example shown, it is set to “Continuous.”
- A Dialog box “Select Linetype” will appear
- Click the “Load” button
- Another dialog box “Load or Reload Linetypes” will display. It is from this long list of linetypes that you need to select the specific linetype.
- Click OK.
- The linetype is loaded but it isn’t assign to the layer.
- So now that you are back in the “Select Linetype” dialog box, select the newly loaded linetype and then click OK.
- The linetype is now loaded in the drawing and assigned to a layer
The loaded linetype can also be seen in the Properties palette and in the Properties panel of the ribbon on the Home tab of the ribbon.
AutoCAD Tips 
I’d like to offer an additional tip: The system variable MEASUREMENT controls which linetype file to use (yeah, there are TWO).
If MEASUREMENT is set to 0 (“zero”), then AutoCAD uses the ACAD.lin file (for Imperial drawings), and if MEASUREMENT is set to 1 (“one”), then it uses the ACADISO.lin file (for metric drawings).
If you have a drawing where the linetypes aren’t scaling properly, it could be that someone either set MEASUREMENT incorrectly, or, they use an Imperial vs. Metric template by accident.
If you need to fix the drawing, set MEASUREMENT accordingly, and then simply reload the linetypes.
-Murray Clack
In the context of assigning one to a Layer, you can also use the -Layer [command-line version, with the hyphen prefix] command, choose the Ltype option, and just give it a linetype name. If it’s in your ACAD.lin/ACADISO.lin (as appropriate — see Murray’s Comment) file, IT WILL FIND IT, WITHOUT your needing to have Loaded it first! (But to get any linetypes in OTHER files into the drawing, you need to use either the outlined procedure or the Linetype command’s Load option.)