I just added phpSuExec to my server and this is what I had to do to get MT to work properly:
1. open MT.cfg and find these lines:
CODE
# When creating files and directories, Movable Type uses umask settings to
# control the permissions set on the files. The default settings for file
# creation (HTMLUmask, DBUmask, and UploadUmask) are 0111; for directory
# creation (DirUmask), the default is 0000. You should not change these
# settings unless you are running MT under cgiwrap or suexec, or some other
# scenario where the MT application runs as you; in addition, you should not
# change these settings unless you know what they mean, and what they do.
#
# DBUmask 0022
# HTMLUmask 0022
# UploadUmask 0022
# DirUmask 0022
2. Remove the "#" from the 4 settings (Starting with "DBmask 0022")
3. Save and re-upload the file.
4. Use a good FTP program (like Flash FXP or SmartFTP) to connect to your server. CHMOD the folder that has your ARCHIVES in it to 0755 (also CHMOD all the files/folders benath it to 0755).
5. Once that is done it should all work correctly. If not you might try changing the "HTMLPerms" to 0755 in the mt.cfg.
*Pretty much when you turn on phpSuExec it means that php scripts cannot run as "nobody" which in turns makes each site run their scripts as that "username", which means not "everyone" can write to all the files. Simply put, you cannot use 0777, you use 0755 instead, or you will get 500 errors.

~Riika