This issue is one of LDD's most prominent shortcomings. Unlike other CAD software, the simplifications that LDD makes in exchange for ease-of-use restrict some creations from being assembled with any degree of ease. However, the treads in this model are possible to apply, although I cannot say how easy it would be to get them the way you would want.
A quick search from more experienced LDDers on YouTube turns up that the process is very redundant and tedious. Each link must be adjusted individually, and when needed, certain gears must be turned to accept the chain. However, getting the initial link on the system requires a crafty Technic device used to mount a five-hole tread to a bar suspended just above the gear in question. From this five-hole link, the smaller links can be added, as shown in this video.
I tried the process myself and while it is painfully difficult, the end result is achievable. Here is a photo with the Technic system in place, and another after I removed it and added the correct link.
Of course, I had to try this in real life where dexterity is on my side. I ran up the exact construction as I see it in your provided picture and have determined that the perimeter of this design falls just over an even multiple of links. Twenty-five links are just barely too short to connect at the ends, and twenty six is much too baggy to stay seated on the gear teeth.
For another example of adding treads in LDD, you may also want to see this time-lapse video. Hope this helps and happy building.