It seems like I have stumbled across a possible solution. Here it is in case somebody else experiences the same issue. The solution may actually apply to all kinds of load freeze issues ("Loading Asset Bundle Definitions"), but I cannot verify this.
The problem seems to be the permissions. It is necessary to set the permissions of all (?) files and folders within the KSP_osx folder to read, write, AND execute.
The solution can be found on the page "Known Issues & Self Help" (http://forum.kerbalspaceprogram.com/index.php?/topic/83211-known-issues-amp-self-help/) if you scroll down to the section "Mac OSX Specific Issues".
Please note, however, that apparently you have to use the terminal command as it is given there ["open a Terminal, change the directory to where the KSP_osx is and type: "sudo chmod -R 777 KSP_osx" (without the quotes)"]. Setting the file permissions from the graphical user interface (as suggested on that page and in the referenced link there) does not work, at least for me. We had already tried that without success. The reason may be that the graphical user interface does not give you an option for setting--or even just viewing--the execute permission.
Possibly just the KSP.app needs the execute permission while read&write may be sufficient for all others, but this is just speculation on my part. Why this would not be set to "everyone" for an executable by default upon installation I do not know nor understand.
One final note of caution: We have not extensively tested the program with the new permissions, yet. It does load completely and it does start. Still, there may be other issues with this path during gameplay.