That is your response has to look like this, watch the lines 18-21 and 32-35.
1 <?xml version="1.0"?>
2 <methodResponse>
3 <params>
4 <param>
5 <value>
6 <array>
7 <data>
8 <value>
9 <struct>
10 <member>
11 <name>categoryid<name>
12 <value>1</value>
13 <member>
14 <member>
15 <name>titlename>
16 <value>Category 1value>
17 <member>
18 <member>
19 <name>descriptionname>
20 <value>Description 1value>
21 <member>
22 </struct>
23 <struct>
24 <member>
25 <name>categoryidname>
26 <value>2</value>
27 <member>
28 <member>
29 <name>titlename>
30 <value>Category 2value>
31 <member>
32 <member>
33 <name>descriptionname>
34 <value>Description 2</value>
35 </member>
36 </struct>
37 </value>
38 </data>
39 </array>
40 </value>
41 </param>
42 </params>
43 </methodResponse>