XE 10g had native Ubuntu support.
XE 11g does not support Ubuntu, according to the official documentation (see links below).
The big reason I discourage using an unlisted/unsupported platform is: after install, should any issue occur, you will never really know whether that issue is a result of the platform mismatch. In other words, it might have been avoidable, and the problem might be a direct result of your decision.
And if you don't have the experience and background to do the install on your own, how do you expect to support that unsupported environment, possibly in production. That is a heck of a risk to place on your application and potentially on your customer/employer.
So, while I still strongly discourage installing XE11g on Ubuntu (or any other "unsupported" platform) at least I can point you to a decent resource so you have a chance to get it right. From the Oracle Forums:
If you have any issues with XE11g, here are some of the better resources I've found:
Oracle's OTN XE Portal
Oracle Database 11g Express Edition Documentation
Forum: Oracle Database Express Edition (XE)
Oracle's OTN APEX Portal
Oracle Application Express Documentation
Forum: Application Express
Hope that helps.