I'm assuming you're using a computer that runs Windows?
If so, there is an application called Character Map. On my computer, which runs Vista, it's under the Start bar menu, All Programs | Accessories | System Tools. Once Character Map is running, you can select characters that have macrons, and cut/paste them into the web browser text box.
Or, try typing your posts into Wordpad, using the following keystrokes for "macron-ated" characters:
Ā = alt-256
ā = alt-257
Ē = alt-274
ē = alt-275
Ī = alt-298
ī = alt-299
Ō = alt-332
ō = alt-333
Ū = alt-362
ū = alt-363
The sequence alt-xyz means that you must hold down the alt key, then type x, y, and z sequentially on the numeric keypad, then release the alt key. You may also have to play with the font in WordPad. Your mileage may vary. Hope this helps.
I, Lex Llama, super genius, will one day rule this planet! And then you'll rue the day you messed with me, you damned dirty apes!