trying to use a blank dvd-r and my mac keeps spitting it back out without even coming up on screen, why is this happening

If the mac doesn't accept the disc means that the drive probably isn't the right type.

Apple offers two types of optical drives: the combo drive that reads only DVD-ROM (reads and writes various types of CD) and the SuperDrive that reads and writes DVD+R and DVD+RW besides CDs, also supports double layer discs).

Probably your mac has a combo drive and it doesn't accept DVD-R discs.

To make sure, you can check it in the box your mac came in or in the profiler in the system.

Profiler is a program that lives in the Applications -> Utilities folder (complete name is "System Profiler"). It shows abundant info about the machine; in the left panel you can choose "Disc Burning", and in the right panel it shows the kind of drive you have and the types of discs it accepts.

Also, some manufacturers like being cute and making disks colored, those are hit and miss in a mac and should not be used some older PCs can have fits with them as well. Kinda rare for DVDs but it does happen.