You probably tried to import it as a set of instructions vs. a code-block. The Import processes the import differently based on whether there are code-block (e.g. $PRGRM/$PGMEND). For example, if you are just inserting instructions, you MUST provide an address to "insert them" at. This is not needed if the text file contains a bunch of code blocks, not a list of single instructions.
In the Insert Instructions dialog, just change the Instruction Import Method from As Rungs of Logic to As Code Blocks. That should make it work!