You need to edit line 1 of the dat files first to remove the test before the 0 as the part designer won't load what it considers invalid files.
For example
test0 MINI FIGURE, WIG NO. 18 (Needs Work)
Needs to be:
0 MINI FIGURE, WIG NO. 18 (Needs Work)
For example
test0 MINI FIGURE, WIG NO. 18 (Needs Work)
Needs to be:
0 MINI FIGURE, WIG NO. 18 (Needs Work)