More often than not Internet Download Manager refuses to resume downloads due to expired links and you risk the download restarting or failing to pick a new link from the site. This usually happens when your download is at 99.9% which is quite annoying.
Here is how to go about .
go to the IDM page and select the download you want to resume.
Right click on it and select properties. From the dialog box select referrer and copy the link.
post the link in your browser. This should open the download page of your download.
Let IDM capture the download as you would do usually then instead of selecting download now choose download later.
Now go back to your new download in IDM and select it. Right click and select properties.
select Address. Right click and choose select all. With it highlighted right click again and choose copy. You may use Ctrl+C for this step.
Now select your old download and right click and choose select all. With it highlighted right click again and choose paste the new address you copied from the new download.
Close the dialog and Resume. 😉