Use Chr(10) and Chr(13) when you want to start text on the next line, e.g. use the expression:

="First line of text" & Chr(13) & Chr(10) & "Second line of text"

The problem here is when you export to Excel you get a box for Chr(13)

This is fixed by leaving out the Chr(13)

="First line of text" & Chr(10) & "Second line of text"