Importing a DWG or DXF file into PartyCAD is done via a screen grab as explained in the Picture as Plan video.
This method required that you display the file contents on your screen using a DWG/DXF viewer program. If you don't have the software to do this, one place to look is at the free DWG/DXF viewer programs from AutoDesk... A360 Viewer and DWG TrueView.
If those don't suit you, use Bing or Google and search for: CNET Free DWG Viewer
Get the software you choose up and running, the load your DWG/DXF file and maximize the screen and zoom in on the part of the DWG you want... look for Pan/Zoom controls. Often you zoom using the scroll wheel of your mouse and pan by dragging with your mouse. Make the part of the plan you want as large as possible on the screen. If you like, there is usually an option to set the background color to white.
Once the image looks as good as it can, the rest is easy... see the Picture as Plan video if you've forgotten what comes next.
Another way to go
A much easier way to handle any AutoCAD drawing in PartyCAD is to have it sent to you not as DWG or DXF file, but as a PDF. AutoCAD allows the user to work in either paper space or model space. In paper space, the background is white, like a piece of paper. AutoCAD also allows for the selective saving of layers to a PDF files. So what you should ask for is a paper space PDF file containing the structural layers of the plan. This should get you a file which will be easy to work with and which gives a nice result in PartyCAD.
Notes
If the resolution of the image is not good enough for you, use multiple screen grabs of the zoomed in PDF then assemble these to make a high-res image in a paint program.
Another way to go would be to use Google Chrome to open the PDF, then right-click and use Save as... Haven't tried this myself, but should work according to the internet.