"this nook cannot read this file" on .apk

I got a similar problem with Nook Tablet wit 1.4.0
I have managed to download an app so that my settings have been changed to accept those apps and sideload
If i go to another app market, I do install apps.
But for some reason I am limited (Amazon Apps refuses to work because I am not in the US and others like AndroidPITs have limitations which I dont understand, as on a chinese android pc they work, however not on this nook, for example Skype can not be found)
So I'd like to install from sd card
And i get this message "this nook cannot read this file"
If I mail to gmail, in nook mail I cant open attachment (nor any others)
Any workaround?

Thanks for your reply!
I have found a quite satisfactory way without roothing:
- I used the trick posted on xda-developer forums (and around) to sideload apps
- I used another trick provided there to use golauncher Ex as a launcher instead of nook, and get the system use it as default home add a nook home button onto it
- I finally managed to get Nook open .apk: not though the nook file explorer, but using ES explorer instead.
So it is quite satisfactory since I had a number of apps I could load from a SD card, for others I use alternative app centers,; unfortunately I can't use amazon apps for some geolocalization mlimitation (I am in Lesotho) altough it workds from here from a Chinese tablet...
Agnes