Older Flexi versions may store levels in one of two manners:
> In "levelXXX.dat" files
> As LVL files in a "levels" subfolder
The latest version on the other hand, stores them in a "levels" subfolder, but with arbitrary names (and a "levels.ini" file to store the information on level ordering). When updating an older pack, if it uses the LVL files format, there is no problem. If however it uses "levelXXX.dat" files, generally an error message will occur saying something like Cannot Create File "****\levels\0101.lvl".
(Until an update that fixes this is released, the workaround is to create a folder called "levels" in your project folder, then try again - then it will work. The bug occurs because the "levels" folder doesn't exist, and the toolkit doesn't create it; with projects using LVL files, the folder does exist so there's no problem.)
Finally made a fix for this; should have it up soon (just going to check if there's anything else for Flexi that I can address now, first).