First of all, a variable name cannot be a number, so you'll need a different way of getting the frame numbers.
Secondly, you don't need to change the Var of a text box to change its value. You can just say:
myTextBox.text = "some text";
My suggestion would be to create a number of objects to hold your data and store them in an array. Each object should contain two variables, a frame number and a string for the subtitles. Each frame you check to see if the current frame matches the next object's frame number in the array. If it does, set your text box value to the corresponding subtitle string.
As far as loading multiple variables from a text file, I can't really remember off the top of my head. But I know that you can write more than one into a file like so:
?var1=10&var2="hello" etc... and read it in. (using loadVariables I think)
In that case you could do something like this with your text file:
?frame1=20&sub1="this is line 1"&frame2=50&sub2="this is line 2"
and so forth.
Read up on the forums and the help about the loadVariables() function.
hope that helps