You don't mention what country you are from / what carrier / if your phone was purchased in a different country. All these may have a bearing on whether the update is released to your particular set up.
The first thing I would try tho is using Sony Ericsson Update Service instead of PC Companion, downloadable from this link
http://www.sonyericsson.com/cws/support/phones/topic/1.433308/xperiax10?cc=gb&lc=en
Let us know how you get on.
