Few things need to be confirmed first:
1. This MP4 video is playable on computer;
2. The SD card is under good condition and can be used without problem
3. The video is under 4 GB size limit because of FAT32.
If you want to transfer your videos to Galaxy Tab 10.1 from your PC computer, you need to connect your tablet to PC first via USB. Here is the step-by-step guide:
First, connect the two devices using a USB cable.
Secondly, after connected the cable to both devices, your PC computer will install the drivers to facilitate the connection.
Thirdly, the tablet instantly is mounted as another storage device. Now you can copy media flies/document to Galaxy Tab 10.1 from PC freely.