The option of database type is "SQL Server" for Microsoft SQL Server. If you can't see this in install.page, check the install.htm file.
<jforum_installed_path>/templates/default/install.htm:
<tr>
<td class="row1" align="right" valign="top"><span class="gen"><b>${I18n.getMessage("Install.dbType")}: </b></span></td>
<td class="row1" align="left">
<select name="database" onchange="changeDbPort(this.value);">
<option value="mysql" <#if database?default("") == "mysql">selected="selected"</#if>>MySQL</option>
<option value="postgresql" <#if database?default("") == "postgresql">selected="selected"</#if>>PostgreSQL</option>
<option value="oracle" <#if database?default("") == "oracle">selected="selected"</#if>>Oracle</option>
<option value="hsqldb" <#if database?default("") == "hsqldb">selected="selected"</#if>>HSQLDB</option>
<option value="sqlserver" <#if database?default("") == "sqlserver">selected="selected"</#if>>SQL Server</option>
</select>
<br />
<span class="gensmall">
<font color="green">${I18n.getMessage("Install.dbTip")}</font>
</span>
</td>
</tr>