To endorse profiles, you need to use Standard LinkedIn.
Go to Dux-Soup Options, Actions tab and turn on endorsing function:
Then go to your LinkedIn and filter your 1st-degree connections that you wish to endorse.
Once you are happy with your filtered list, click on Visit Profile to start endorsing.
Please note that if you wish to endorse only, you will need to turn off other functions (like direct messaging) to avoid mixups.