consegui achar outro tutorial de como instalar o xml dom... ( o comando no linux centos é "sudo yum install php-xml" e depois "sudo service httpd restart" )
porem agora aparece o seguinte erro...
Error: PagSeguroServiceException
Message: [HTTP 401] - UNAUTHORIZED
File: /var/www/html/addons/donates/lib/service/PagSeguroPaymentService.class.php:74
File Line Function/Method /var/www/html/addons/donates/lib/domain/PagSeguroPaymentRequest.class.php 600 PagSeguroPaymentService::createCheckoutRequest() /var/www/html/addons/donates/modules/doa/process.php 40 PagSeguroPaymentRequest::register() /var/www/html/lib/Flux/Template.php 375 include() /var/www/html/lib/Flux/Dispatcher.php 170 Flux_Template::render() /var/www/html/index.php 180 Flux_Dispatcher::dispatch() Exception Trace As String#0 /var/www/html/addons/donates/lib/domain/PagSeguroPaymentRequest.class.php(600): PagSeguroPaymentService::createCheckoutRequest(Object(PagSeguroAccountCredentials), Object(PagSeguroPaymentRequest))
#1 /var/www/html/addons/donates/modules/doa/process.php(40): PagSeguroPaymentRequest->register(Object(PagSeguroAccountCredentials))
#2 /var/www/html/lib/Flux/Template.php(375): include('/var/www/html/a...')
#3 /var/www/html/lib/Flux/Dispatcher.php(170): Flux_Template->render()
#4 /var/www/html/index.php(180): Flux_Dispatcher->dispatch(Array)
#5 {main}
Seu e-mail e token estão configurados corretamente ?