Non-ASCII characters (basically, non-keyboard characters) are represented in the AmVerse texts by "character entities" that consist of a string of characters beginning with an ampersand and ending in a semicolon, like this:
ö ( = o with umlaut)
— ( = a dash as wide as an "m")
è ( = e with grave accent)
In A/E these are displayed in little boxes.
Most of the characters you'll need are already built into the A/E rules file and may be inserted using "Insert Entity" on the A/E "Entities" menu (hotkey = Ctrl-E).
The confusing thing about A/E's "Insert Entity" option is that the initial list of entities it presents you with is very short: it contains only the entities already used in that document (as well as those especially defined in that document, if any). This is the entity set that it calls "Local and Active."
In order to see other available entities, pull down the "Entity Set" menu at the bottom of the "Insert Entity" screen: you'll see other available entity sets that are invoked by rules file: the sets called "latin-1", "latin-2" "numeric" "publishing" and "med-sel1" (the last of these is a sort of anthology of characters drawn from many of the ISO entity sets).
These should contain most of the characters you're likely to run across.
Unfortunately, the names are often quite cryptic. How can you know what the entity is called for (say) an "ae" printed as one letter, or a "c" with a cedilla, or a Spanish "n" with a tilde? One way to get some help is to do a "search in files" in TextPad (Ctrl-F5), searching the files *.ent in directory Work/Markup/Amverse/Code/Entities, with the search set to display "all matching lines" and the regular expression feature turned off. If you search, say, for "ae", you'll get the following hits, among others:
isolat1.ent(23): <!ENTITY aelig SDATA "[aelig ]"--=small ae diphthong
(ligature)-->
isolat1.ent(24): <!ENTITY AElig SDATA "[AElig ]"--=capital AE
diphthong(ligature)-->
med-sel1.ent(12): <!ENTITY AElig SDATA "[AElig ]"--=capital AE
diphthong(ligature)-->
med-sel1.ent(13): <!ENTITY aelig SDATA "[aelig ]"--=small ae diphthong
(ligature)-->
From which you can immediately see that the "ae" digraph symbol should be called æ.
Searching for "tilde", you get these hits (among others):
isodia.ent(24): <!ENTITY tilde SDATA "[tilde ]"--=tilde-->
isolat1.ent(19): <!ENTITY atilde SDATA "[atilde]"--=small a, tilde -->
isolat1.ent(20): <!ENTITY Atilde SDATA "[Atilde]"--=capital A, tilde-->
isolat1.ent(45): <!ENTITY ntilde SDATA "[ntilde]"--=small n, tilde-->
isolat1.ent(46): <!ENTITY Ntilde SDATA "[Ntilde]"--=capital N, tilde-->
isolat1.ent(55): <!ENTITY otilde SDATA "[otilde]"--=small o, tilde-->
isolat1.ent(56): <!ENTITY Otilde SDATA "[Otilde]"--=capital O, tilde-->
isolat2.ent(57): <!ENTITY itilde SDATA "[itilde]"--=small i, tilde-->
isolat2.ent(58): <!ENTITY Itilde SDATA "[Itilde]"--=capital I, tilde-->
isolat2.ent(119): <!ENTITY utilde SDATA "[utilde]"--=small u, tilde-->
isolat2.ent(120): <!ENTITY Utilde SDATA "[Utilde]"--=capital U, tilde-->
From which you can see that the entity name for an "n" with a tilde is in fact ñ.
If you know the name of an entity, you can type it directly into A/E as (e.g.) "ñ" but it is probably safer to use the "Insert Entity" option.
If you need a character that is not available in the rules file (i.e., not available in any of the entity sets available to you in A/E), let me know.