Robert,
Here is a link to the actuator assembly file in Dropbox.
https://www.dropbox.com/s/vs45ld4o9w...uator.zip?dl=0
Here is the workflow I used.
1. Open the TXT file in Notepad.
2. Click File > Save As.
3. In the Save As dialog box, select All Files from the Save as type: list. The default is Text documents.
4. Enter the File name with a STP file extension. In this example, Dart Linear Actuator.stp.
5. Click Save.
6. Start Autodesk Inventor and open the STP file. This will take a few minutes.
7. Save the assembly. The files are saved in a folder named Imported Components.
Let me know if you have any issues or more questions.