Small update, I installed unetbootin from slackbuilds.org and I was finally able to create my usb key. The one I installed before came from pkgs.org, the packages are already compiled.

I apologize for being late to the party but for future reference you may get some help or at least some certainty from IsoMaster, also on Slackbuilds.org, that both informs the user of the existence and validity of the boot record and allows installation and manipulation of them if it is absent.

It is not recommended to download packages from pkgs.org since they could be compiled against different libraries than what you have in your system and it won't list any dependencies (since they aren't built into the packages themselves). It's typically safe to download from trusted repos like Alien Bob's and rworkman's, but for relatively easy packages to compile, it is highly recommended to build them yourself to help ensure it will work on your system.

Also, all scripts on SBo should work with the version of Slackware they're designed for (and the default is 14.2 right now).