SD slot for mac mini
The Mac mini, since Mid 2011, has one Secure Digital (SDXC) card slot.  Prior to that, the Mid 2010 models have one Secure Digital (SD) card slot and prior to 2010 the Mac mini has no card slot.

Back side of Mac mini (Late 2014):



If the flash memory card is a micro card, then to use the SD/SDXC card slot, you need an SD card Adapter as shown in the example image below.



If you do not want to use the SD card slot then you'll need a USB SD Card Reader. As an example, the Insignia™ - USB 2.0 SD/MMC Memory Card Reader - Black, which is compatible with, SD, SDHC, SDXC, microSD, microSDHC, microSDXC, MMC and MMC+ memory cards on PC and Mac, shown in the image below.