Read/write to XML file in data folder

[arek1]

Yes I'm talking about public form but have success - it now working OK in my template :-)
I was made all the time so simple error it is shame to write about this.
Again thank you for your patience.

Back for moment to plugin. Reading Wiki I hope I understand hooks (can decide to what part of page plugin code will be added but can't understand filters (as I understand using filters I can decide on what pages plugin will be displayed).
I can't understand how to use filters to put plugin code only on one particular site.

Best
ArekS