member since

[Max Hodges]

is there a place to see a member's registration date? i.e. "member since mm/dd/yyyy"?

 

[Martok]

Can I show member since on topic page like this, under my avatar?

Yes. Go to ACP > Appearance > Style Properties > Message Elements and tick the option "Show author registration date".

 

[Digital Doctor]

If Martok's suggestion doesn't work (seems like it will).
Ask here about style tweaks.
http://xenforo.com/community/forums/styling-and-customization-questions.47/
The area you talk about is called the Postbit.

 

[Paul B]

The area you talk about is called the Postbit.

Not in XenForo it isn't.
It's called the User Info Block.