Lappro

If you aren't that skilled with computers then i would just advise you to get a proper PC. Unless you manage to find one already on the internet. I think it is not very likely someone will just do the work for you, so you either have to get it working yourself or find a premade solution.

just did a quick search, this might be the thing you are looking for: http://www.joshondesign.com/2011/09/22/hidden-treasure-appbundler/

in the technic launcher on tekkit classic you have options: manual build selection -> 3.1.3 version voila. edit: without the mean stuff... eventhough i was the only one that gave a real answer unlike all the other posters here including cheapshot himself...

just a question: did you install java and/or java SDK? Because if you don't have that you cant run .jar files. Another thing that could be the problem: Recently some serieus bugs/leaks were found in java and as a reaction Apple forced all their computers to disable java completely without giving their users a choice. So that could also be the problem. Since I don't have a mac (and don't want one) I can't check that last statement, but that Apple removed java from all their systems is a fact, but it may only be related to java run from the browser and not java running at all.

just a check: did you upload the zip to the internet in a way that it is directly accessible and used that link to add it to the launcher as custom zip? just like my lappro.wrongplace.be/mods.zip which i use for optifine.

you haven't really posted much relevant information like how your custom zip looks like. At the moment it looks like all you are saying is: make the zip for me since i dont have a clue and want to look into it more. That will get you nowhere.

you have the modpack.jar in the bin folder (inside voltz folder which you should be able to find). copy that for your custom zip and install anything in it like you would normally install to the minecraft.jar. That should probably work. I did the same for optifine which needs to be installed in the minecraft.jar.

since there is a "open to lan" option you never know when your solo map becomes public ;)

As i found this was a problem aswell i made myself a custom zip to add optifine to my game. if there are other lazy people like me you may use the zip i host on my mini server. No garantuees of uptime or anything. It just adds optifine version: OptiFine_1.4.6_HD_U_B4 It works with tekkit lite version 0.5.2, don't know about others. link: http://lappro.wrongplace.be/mods.zip