Movie Edit Pro (MEP) and Video Pro X (VPX) do not support sub titling using srt, or any other subtitling format, files.
You would have to use Titles or captions which would be permanently visible and if making DVD or BD discs there is no option for having a sub title menu on/off.
If making DVD or BD discs there are 2 options:
Using MEP or VPX create a multi movie project, see this tutorial for how to do this (it refers to an earlier version of MEP but is applicable to the current version and VPX, and create two or more movies within the project, one without subtitles and one or more with in the required languages..
OR
Use a program such as DVD Architect to assemble and create the disc.
Low cost free and or paid version (20$) using subtitle is Divx software also you can use a free version to create your subtitle like Aegisub. Also you can use special DVD and BR creator using subtitle and sound track using multi language that you can activate using the player.