When you pull your AXF into ArcPad Studio, you should see the layer whose form you want to edit listed under "Feature Layers". Double click it to bring up that layer's customization. Under "<FORMS>" you should see one called
<EDITFORM>YourLayerName
and possibly another called
<FORM>Editform (if you renamed it to editform)
Here's what I'd try:
Click the red and blue arrow icon on the toolbar to change the view to XML.
Near the top, you'll probably see something like this:
<ArcPad>
<LAYER name="whatever">
<FORMS>
<EDITFORM .... various things
<PAGE .... various things
Scrolling down, you need to find the form you made. It will be listed as a regular <FORM ....
Find where the regular form closes itself, this looks like </FORM>
Now, copy everything between the <FORM> and the </FORM>, but not the lines with the <FORM> / </FORM> on them. (The first and last lines should be <PAGE> and </PAGE>)
Paste this selection into <EDITFORM> section, overriding everything between <EDITFORM> and </EDITFORM>.
Save, now click the red/blue arrow icon to get back to form view, and double click the <EDITFORM> icon, if it shows your custom form, you're good to go, and it will open when you edit a feature.
Hope this helps,
Eric